This is much more well-put than I'd have managed to write. Had a couple years of startup experience and I feel like the instability and "wrong lessons" were too

This is much more well-put than I'd have managed to write. Had a couple years of startup experience and I feel like the instability and "wrong lessons" were too much for me personally. IMO a lot of people conflate working in a startup with the experience of being a more senior engineer becoming a founder or early employee with similar levels of autonomy. I worked in roles during my first ~5 years where I was the 4th or 5th engineer and was basically given narrowly-defined tasks to complete, with no great guidance on accomplishing them, and with my coworkers too busy to document or explain how to do basic things like run parts of the codebase locally, connect to the db, etc. You can get a lot better mentorship in those more "boring" traditional jobs at larger companies. You can get that in a startup environment too, but depending on the startup the degree to which that's actually something people prioritize and have capacity for varies wildly. I guess I'd recommend trying smaller startup environments if you happen to already know everything about their tech stack and/or have a decent amount of experience as a SWE already. Both are probably useful but I've personally gotten a lot better ROI growth-wise when working on an established team with stable goals, time to mentor, document, write tests, etc.