I just flash LineageOS even when the device is currently supported by the OEM. I buy the hardware from them, not the software.
Edit: Like, the vast majority of actual custom ROMs users are either using Pixel Experience or LineageOS, there are a bunch of other ROMs, but those are mostly “purpose built” for enthusiasts of what they offer. Like, GrapheneOS is for security reasons, and things like that.
There is a bit of headache installing custom ROMs, but once you install it, it is usually pretty stable. Also, I don’t get the locked vs unlocked bootloader thing in regards to security. The device is stolen and outside your hands, it is doubtful that a thief would go through the steps of flashing a ROM, but wouldn’t be smart enough on how to make the device unusable if it had a bootloader locked. Either way you are screwed.