When it comes to the battle for Crimea Russia has already taken a huge defeat. Morale will be rock bottom, HIMARS will sink that moral even further.Crimea is a large area but only connected to the mainland by a narrow bottleneck where Russia have been making their defences ready for the best part of a decade. Worse, even if they can punch through that first bottleneck there's another a few dozen kms further in. Then, even if they can punch through the rest of Crimea, Russia has a large military base at Sevastapol making an ideal place to mount a rearguard action from. Meanwhile, Ukraine doesn't have the sea power to mount an amphibious assault, whilst Russia does have the sea power to keep weapons and soldiers flowing into the region even with its bridge connection severed.
It's difficult to see how Ukraine can take Crimea back at all, let alone as an early target.
Maybe they conscripts will run, leaving just the separatists. Who knows but to get to fight for Crimea does mean the Russians are very much on the back foot.