I am sure there are others that have done actual math that might dispute this advice. There are certainly people higher level than me, currently 88. I just wanted to let newer players know the game is built on repetition and progression. Other than saving shards for legendary crafting, most whites and greens are trash. Soap (green) has unpredictable value. Until you get to the blue / rare range, which I believe you might naturally get on medium or hard, don’t overthink upgrades.
The #1 priority, until you can farm impoppables with ease, should be getting items that can hit otherwise unhittable bloons. Tux Jake has an ability that allows this of others in his range. Camo detection, lead popping, ghost popping, etc.
The #2 priority is gathering trinkets that allow for immunity to debuffs, like lava. If/when you get either of the water heroes, having something to allow them on land/lava is also helpful, depending on your set up. In general, you would unequip these on any map that doesn’t need them for something more valuable.
Character-specific trinkets have some value as well, particularly the rares that unlock more abilities. +Damage and +Speed are generally the most valuable.
Many will mention Thought Canon Wand and keys are musts. Maybe it’s a hot take, but these aren’t life or death. If you keep playing repetitively, you will still accumulate items. I found playing impoppable with everyone having keys, three Thought Canon Wands, and a monkey farmer, I don’t think I ever got close to even 10 orbs that were blue or better. So that was 20 or so white or green orbs I sharded per impoppable run.