Earlier quoted context omitted.
Something with meaningful restrictions such as register count and RAM that you'll hit quickly when hand-coding assembler is a super IMHO useful teaching tool. 16 registers is luxurious!
For a teaching tool, you can get the best of both worlds by simply restricting RISC-V to fewer registers. Something like: "Write this in RISC-V assembly, but only use registers 0..3, and 31 for the return address."
rv32e and rv64e have 16 registers rather than 32.
Maybe use these.