You’re probably not going to like this, but I think a lot of your best options are Pixel phones of various kinds. A lot of the phones with unlocked bootloaders are indie phones that are quite expensive (largely because they don’t have the benefit of economy of scale that, say, Samsung or LG or whoever enjoys) and the significant majority of regular consumer phones are really locked down and nearly impossible to replace the OS with anything not signed by the original manufacturer. But Google Pixel phones are some of the few counterexamples – phones that have unlockable bootloaders despite being made by a big evil company.

I wouldn’t recommend the stock firmwares on such phones, of course. But with a custom firmware, you can still “degoogle” as far as software is concerned.

Beyond that, be sure to do your homework. In particular, make sure that VoLTE is supported on the particular phone/OS combination you want to use or it won’t be able to make voice calls.

Yeah, I’m also planning to switch to PostmarketOS. I was running Ubuntu Touch when my carrier changed things to where my phone didn’t work as a phone without VoLTE. Ubuntu Touch doesn’t support VoLTE on my Pixel 3a so I reluctantly switched to stock.

It wasn’t until after I switched to stock firmware that I found out that PostmarketOS has experimental support for VoLTE on my device. I’m hoping “experimental” is good enough for my purposes, but we’ll see when I get around to doing it.