Rome wasn’t built in a day… Also, the community is remarkably creative if they desire. I’m sure there will be an OpenOS or LineageOS port in no time. As a first step, unlocking the phones bootloader is apparently still dead easy, as was mentioned by Tweakers:
Loosely translated, it’s a piece of cake to unlock the bootloader. In the “developers options” menu you can enable OEM unlocking, then reboot in fastboot mode and use the fastboot tool on a connected laptop/pc to unlock.