ARM64 also has fixed length 32-bit instructions. Yes, immediates are normally small and it's not particularly orthogonal as to how many bits are available.

The largest MOV available is 16 bits, but those 16 bits can be shifted by 0, 16, 32 or 48 bits, so the worst case for a 64-bit immediate is 4 instructions. Or the compiler can decide to put the data in a PC-relative pool and use ADR or ADRP to calculate the address.

ADD immediate is 12 bits but can optionally apply a 12-bit left-shift to that immediate, so for immediates up to 24 bits it can be done in two instructions.

ARM64 decoding is also pretty complex, far less orthogonal than ARM32. Then again, ARM32 was designed to be decodable on a chip with 25,000 transistors, not where you can spend thousands of transistors to decode a single instruction.