The Xperia phones are often horrendously locked down and don’t provide bootloader unlocks all the time.
I would definitely recommend a Pixel device if you’re going to go De-Googling. That, or go select your desired ROM beforehand and buy whatever they support the best. You can find out if you look into Graphene or Calyx or Lineage as examples for which devices they support the best right now. Buy it unlocked, and unlock your bootloader.