Smaller instruction set and shorter reference manual aren’t necessarily good attributes. The risc-v reference manual is short because in part because it is has terse-to-vague descriptions of things (not great for a specification) and because the ARM ARM specifies a bunch of things which are omitted from the RISC-V architecture spec, which either implementors will have to design themselves or the RISC-V org will eventually add in a revision or extension spec (e.g. Zicbom, Zicboz, Zicbop)