The thing is in such a case secureboot doesn’t help and is unnecessary. Secureboot only does anything for the concept of “trusted suppliers”.

If the system has available signing keys for itself, well, hypothetical malware could sign itself using those same keys The OS security mechanisms are the only things protecting that, and in which case the signature validation is redundant.

You can have trusted boot, e.g. LUKS volume sealed to TPM PCRs, but secureboot just doesnt make sense as a mechanism for a user to only trust themselves.