As already mentioned, postmarketOS is probably closest to what you want for an open source android replacement, though it’s still not 100% there if you’re looking for a perfect alternative.
Personally I’m going to try it out as a daily driver when I can get a cheap pixel 8 when the 9 comes out later this year
And it works insanely good on the OnePlus6. I‘m working on that rn. If you want „perfect“ without spending money (donating) or contributing, I suggest proprietary stuff though. Not because I believe its good. I just want to show people that „perfect“ is not a healthy demand.