Custom OS isn’t going to address the anaemic hardware, nor do I think relying on open-source custom ROMs for a niche item is the best way to ensure any hardware-level vulnerabilities are covered.

Not only would it give “anemic” hardware new life, I can point at how its already been done at another in home device. Routers. DDWRT/OpenWRT/Tomato do exactly that for old, otherwise useless routers.

Literally every single argument you make can, and has in other devices, be addressed with a custom OS/Firmware that is designed for purpose without all the bloat and other BS.

You can adamantly say “Nuh uh!” all you want, but it doesnt change the facts.