All instructions occupy the same amount of space in memory.

Both ARM and RISC-V have compressed instructions. Dunno how ARM works but with RISC-V the 16-bit instruction set is freely interspersable with the 32 bit one, which also get their alignment reduced to 16 bits. Gets like 95% of the space reduction possible with full variable-width instructions without overcomplicating the insn decoder.

As to addressing and loads and arithmetic: No such instructions, but every CPU but the tiniest ones are expected to do macro-op fusion for things like indexed loads. Here's an overview.

The MMU thing... well the vector extension can do gather/scatter, I guess it could stay within the letter of "use the MMU once" but definitely not the spirit.