> Expanding it into μops during the decoding stages seems straightforward.

I wouldn't say so, because if you want to be able to crack an instruction into up to N uops, now the second instruction could be placed in any slot from the 2nd to the 1+Nth and you now have to create huge shuffle hardware tk support this.

Apple for example can only crack instructions that generate up to 3 μops at decode (or before rename) anything beyond needs to be microcoded and stall decoding other instructions.