Can attest to this experience as well.
When I was in high visibility roles in start-ups, I dreamt of a place where I could fall through the cracks and kind of breathe -- or at least what I thought would be breathing. I found that spot, and after a couple years feel like taking the offer was a monumental mistake in my career. Nothing is a challenge, my output is minimal yet when I actually do produce something, I get massive praise, and yearn for somewhere that I can throw myself into interesting work.
I think that next phase is coming, but I've definitely stagnated a bit and wish I'd spent more time using the required seat time (have to be visibly online 9+ hours a day) doing something fruitful instead of wasting time on HN and occasionally toying with various project ideas.
The soul crush is real in this kind of position, and as OP said, always being "half on" emphasizes the mental toll that all of my output has suffered both personally and professionally. Or I might just be burnt out.