I’m not going to even pretend it’s a mainstream brand (or a decent price depending on model, even when buying secondhand) but the Sony Xperia phones do allow bootloader unlock. Lineage OS has support for some of the slightly older models that arnt getting official support anymore iirc. Although I haven’t messed with roms personally (not super techy compared to probably literally anyone else here) But it is something to keep in mind if you ever see one around for a price you’d be happy with.