TL;DR - I highly recommend doing this move if you can afford to do it (financially and socially).
Where do I live: Central London
What's the biggest sacrifice: I kept my job in the move and took a ~25% paycut in my job and got slightly worse hours. But the biggest sacrifice was being an ocean away from my friends and family. Oh and the clothes dryers suck here :-)
What have I gained: So much. I got to live in a city of 10M+ people with every possible amenity and activity you could ever want. I got to travel to so many parts of Europe that I never would have gone to otherwise. I learned how to make friends intentionally, something I haven't done since college. I learned how to work remotely effectively, something I thought I couldn't do. I learned how another country works day-to-day, which I think is hard to do without first-hand experience. I learned how to travel with last-minute plans (thanks Covid and RyanAir). I learned how much time I can spend alone with my wife in lockdown without going insane (infinite, so far!). In general I learned that we can live almost anywhere if we are determined to do it.
If it wasn't for my friends back in the US, I'd stay in London permanently. But after ~3 years in London we'll probably head back and try to resume our old life in some form. I hope that when I am older I can return, maybe I'll retire in Spain or send my kids to college in Europe.