A C compiler is relatively easy to implement and it's easy to follow its output if you care about the assembly. For tasks like that, where you want just a lightweight structured programming language on top of assembly, C is very good in principle.

I just want to leave out idiosyncrasies like its automatic type conversions, Rust is way way overkill (and difficult to implement).

Rust is meant to be a replacement for C++, and even then, even if we acknowledge its many merits and C++'s horrible disadvantages, it will always have the handicap of being "different". You can organically grow your abstractions by moving from Assembly, to C, to C++, all in small steps. Writing something in Rust requires somewhat a bigger, discontinuous, step. Not their fault, but that's the state of the ecosystem.