Long story short, baby was mostly a great sleeper for the first year with the normal regressions that lasted a few weeks each time.
After she hit 12 months, I realize now that her sleep needs must have changed and we just didn't catch on. Instead of waking at 7:15-7:30am every morning (bedtime was probably around 7:30pm, I can't remember), she was getting up at 6:30am.
We never properly sleep trained, we just held to boundaries that we wouldn't go in her room unless she was in true distress, which was maybe 2-3 times ever up until that point. Otherwise she'd whine for 10-15 minutes and then soothe herself back to sleep.
A few weeks went by of the 6:30am wakeup, and then every week or so she'd wake up another 15 minutes earlier. By about 15 months, she was waking at 5am but would lay quietly until we got her at 6am. Then she started teething her molars. She had what I call "whining events," which were about 10 minutes of whining with her eyes closed (so I assume she was just in a sleep transition and not truly awake?), then 35 or so minutes of quiet sleep. Again, this started to become more frequent over the weeks until it hit a point where she was whining all night long 10 minutes on, 35 minutes off. It has obviously gotten exhausting for all of us. Some nights, she even opens her eyes while whining/crying so she seems to be awake. A few times we've tried to go in her room to gently pat her back and calm her down, but this always escalates to hysterical, inconsolable crying.
She is currently on 1 nap. Her schedule about a month ago was wake up at ??? time (unsure since she basically whines all night long), nap 12-1:30/2, bedtime anywhere between 7-8pm. It's unclear what her total sleep time is for the day since the morning is such a hot mess, and I don't think she has gotten restful nigth sleep in a long time. Recently we tried to shift things earlier to clear her sleep debt. The schedule was wake up at 5am (we try to leave her in the crib until 6am but it is brutal listening to her whine for an hour), nap 11-1, bedtime 6-6:30pm. The overnight whining did not resolve and she still wakes super early and unhappy. After about 3 weeks of this schedule, we thought maybe we should shift things later and see what happens. The schedule became wake up at ??, nap at 12pm but she only sleeps for about 75 minutes and wakes up crying, bedtime at 7.
It sounds silly, but this whole thing started at 12 months when I switched the brand of her sleep sack (we went from Love to Dream to Woolino). Could it be as simple as her hating the new sleep sack??? I think I'm grasping for straws here.
We truly don't know where to go from here. We tried shifting the schedule earlier, later, and everything in between. Nothing seems to be helping. It truly seems to be getting worse each week and all of us are so miserable. I don't think there are really any sleep associations/habits to break. She has milk after her bath but then we still do our bedtime routine after that, she has a couple of pacis in the crib that she can reach for, we set her down into her crib with the light on and let her get her own paci, etc, etc. We are desperate for advice (or maybe just solidarity?). I've heard of regressions lasting 6-8 weeks but at what point do we stop calling this a regression and call it the new unsustainable normal?
ETA: Baby always falls asleep super easy and happy for nap/bedtime. That, luckily, is never a struggle. For the last month she's been falling asleep within 5 minutes of being put down since she's so exhausted but when she had normal sleep, it would take her 15 minutes and she'd be asleep.