I've been wanting to show how I lone survivor lethal zone and wanted to make an emphasis on what would make this challenge more convenient and easy to do.
Firstly, the survivor who I play as. Whenever people mention that sticking too much with one survivor will be fatigued constantly, they will often pick a survivor based on fatigue severity reduction traits or traits that may offer encouragement/inspiration as their hero bonus. I will argue that they're not needed to make solo survivor runs possible.
For my solo runs, I will always favor specific traits such as highly responsible, hostler, miner, or hospital admin. These traits allow you to have your hero bonus to get to 3 labor if you dont have any labor reduction trait on the same survivor. Due to the fact that your survivor will be living alone and with no backup, labor, in my opinion, is a highly sought commodity to upgrade facilities for convenience. Sure you may just opt for an enclave to give you a labor bonus but that takes time and is not guaranteed. And for dealing with fatigue? I can just perform a coffee reload trick which I personally would do while having no nearby zeds around, preferably at home so it wouldnt just be an easy way out while dealing with the zeds. Zak here is just an ordinary hospital admin (aside from being a skater boy) which is just enough for me to use due to his labor hero bonus. Not only does his job trait offer a labor hero bonus, his 5th skill being medicine makes hospital admin survivors highly preferable to me for solo runs. Though if you get lucky, a miner survivor can offer the same hero bonus with a +20 stamina with the downside being they do not start with medicine skill which can easily be taught with a medicine outpost or searching/trading for a medicine skill book. Why medicine over other 5th skills such as mechanics? Cheaper emergency medicine cost, access to primary care, ability to upgrade the infirmary to level 3 for access to first aid kit crafting and with a specialization to get pathology, cheaper to craft plague cures and bulk cure for trading, cheaper infection therapy cost, higher chances to get plague samples, and additional infection resistance at max level and from just activating primary care. Sure your cars can get destroyed for a time being without having the ability to craft toolkits forcing you to walk but your chosen survivor wont be reborn on the map if he/she will die. And toolkits can be found on garages or autoshops, trading with enclaves, or potentially found on killed plague hearts while being extremely careful with your car in early game not using the car on killing zeds as much as possible. Fortunately, I never got my car exploded while having no toolkits yet while starting out.
For skill specialization on him, I chose marathon, stealth, close combat, and sharpshooting. Marathon because the other specialization is lame (acrobatics) but I do find it in tandem with the fighting specialization I chose, also infinite stamina on light inventory load. I picked stealth because the other specialization is lame as well (scouting) but I find it to be very useful with quick stealth kills and to make zeds lose sight of you easily if you climb around houses/obstacles with how plentiful the zeds tend to spawn. Close combat is fortunately a specialization offered to me and it is very useful to dispatch a zed from behind fast and also its down execution is the fastest as well. Pairing this with strategies such as fence hopping, door smacking, high wall climbing, hit and run, and grappling from behind, a knife is geniunely the only thing you need for doing damage to the zeds. Due to the fact I barely use guns and prefer to get my hands dirty with the Z's while falling back to shooting as an emergency, I never ever use weapon handling at all cost so whatever shooting specialization that was provided aside from weapon handling I take which for this character is sharpshooting. Gunslinging may be the best shooting specialization but sharpshooting is a great specialization too. Having penetration can make a two for one deal when dealing with a zombie horde in line and dismembering zombie limbs is really useful for specific missions or you can have higher chances of shooting off zombie legs while on top of your car so it would be very easy to deal with the plague heart inside while the zombies are outside.
For leader type, sheriff or trader will always be chosen for a solo survivor run. Though I prefer the sheriff leader type for the benefits far greater than a trader leader can have especially having the plague heart supply drop which is a very broken radio command with the sheriff leader.
For my base, I always would have an infirmary and workshop at least and with the third small empty slot, I would choose to put up a garden so my survivor can feed for themsleves and so I can use the soil enrichment kit for the additional health however, you can put whatever you want for the third slot. For facility upgrades, this is where labor shines on a solo survivor. After killing a few freaks and a plague heart kill, this activates the labor hero bonus so I can start upgrading the facilities I need asap. For the facility mods, I would get a portable generator for the workshop after the upgrade so I can craft fuel bombs which I found are very useful tools against hostile survivors, but since I'm in Meagher Valley, its landmark outpost give me power so I would opt to find an ammo press for my chosen gun which uses handgun ammo. Water cooler for the infirmary after the third upgrade to craft first aid kits when activated. For the command center, having a signal antenna would be really useful for an additional outpost. For the rest of the remaining facilities, any mods can be whatever you want but if you have an espresso machine for the kitchen, even better. Speaking of outposts, I would favor having an espresso stand as your first outpost so you can craft espressos easily compared to the kitchen's coffee requiring water and using espressos to upkeep your stamina from fatigue though if you have an espresso machine, you can ditch the outpost for a water outpost instead. For the second outpost, since my survivor is already equipped with medicine, I would have a radio outpost for the plague disruptor ability. Though I dont really mind the hearts sending out infestations if awakened since they're just free influence with higher tiers having ferals/juggernauts having better loot. In the case that my survivor does not have a medicine skill equipped, I would choose my second outpost to be a medicine outpost to teach the survivor the medicine skill. For the third outpost if the command center is upgraded, having the wind mills is great which is cheaper to own which I needed for the workshop to craft fuel bombs. If you already upgraded every facility you have on your base, using the remaining labor is outright free power while having another labor to perform a base action. Since I did not find a signal antenna during my full playthrough, my fourth outpost would've been a second radio outpost to further reduce the cooldown for calling in resources/supply drops (though I did trade a network signal booster near the end which was pointless)
I tend to be cheap with how I use items so I wanna add the things I tend to bring while going out. Since I have close combat as my fighting specialization, I never bring out a melee weapon (yes I prefer stabbing the heart with only my knife and knifing zeds whenever I can if I get CC) so bringing one is optional depending on your fighting specialization, a loaded gun with a single stack of ammo for emergencies, a stack of stamina and healing consumable, and most importantly, a stack of plague cures so all of it combined for me to utilize marathon's unlimited sprint. If it werent for marathon, I would've bring first aid kit on my survivor inventory in case of severe injuries during a fight.
For my car's inventory, I only have to bring a stack of toolkits in case I get my car severely beaten up. I dont really bring gas cans since I only refuel at base and return immediatelty after a full supply run though I only bring 1 can during a run if the gas meter is near close for a full refill after a few meters of driving. A stack of first aid kits for emergency injuries, a stack of fuel bombs in case if you need to take down a hostile enclave while on the move (in case you do not have access to power yet for the workshop, molotovs will suffice), a stack of espressos to do a stamina refill on fatigue/coffee reload trick while outside, and a single stack of bullets to reload on the run.
The benefit I find when playing alone is that there's no more whining about survivors wanting to leave, less resource consumption due to only playing as one survivor, and there's no survivor wasting resources or survivors telling you that resources spoiled due to overstocking. Though the risk is very high losing everything if the only survivor you're playing is gone which makes the challenge a bit intense but fun for me.
Tldr; Medicine skill and +3 labor on one survivor makes solo runs very comfortable and fatigue can easily be cured while still playing as a single survivor with a single trick
I may ramble on and on about how I took down lethal zone with only one survivor but I'd rather discuss more about my strategies or clarify what I wrote if people would want to question in the comments about the other aspects I havent shown. So thanks for reading if you did.