So if someone would like to, say, port a CUDA codebase to AMD, you would use HIP for a more or less 1-on-1 translation?

Any card you would recommend, when trying to replace the equivalent of a 3090/4090?

I can't recommend cards, but you are absolutely correct about porting CUDA to HIP: there was (is?) a hipify program in rocm that does most of the work.