Your best bet right now IMO would be flashing PostmarketOS onto a used OnePlus 6, which is cheap, has good specs and none of the battery issues plaguing the Pinephone Pro. That said, it's not 100% ready to be a phone yet- for now its best use case is as a mini-tablet / PDA kind of thing. Really feels like carrying a pocket laptop around, which is pretty fun as a starting point.