Writing a decent compiler for any language is another huge task. Could take you years to learn how to do it and then implement it. Modern compilers are written and maintained by huge teams. See GCC, Clang/LLVM. Unless you happen to be some kind of genius like Fabrice Bellard https://bellard.org/tcc/
In short, if you want to write an OS, itself a huge job, use an existing language and compiler. Be it C, Rust, Pascal, whatever.
Typically one does not write a "prototype" operating system in a high level language like C and then rewrite it in assembler. Operating systems have not been written in assembler since the 1960's. It would be a huge lot of work for useful reason.
You will almost certainly need to write some small parts of an OS in assembler. See the "Bare Metal" section of this forum for all kind of discussion on writing ARM assembler. There are ARM assembler tutorials all over the net.