I don't think the propaganda about fighting Nazis would be the big problem now. Just declare that you destroyed all military equipment (demilitarization) and killed all Nazis, maybe dig in Donezk and Luhansk Oblast which Ukraine might even begrudgingly accept and declare mission accomplished, despite under high losses.
But the propaganda problem is, that they made a huge deal of NATO support for Ukraine and started to talk about being actually at war with NATO, already fighting WW3 now etc. This part of the propaganda was great to explain Russian losses, but really makes it hard to retreat, as now it looks like they have been beaten by NATO. That's a whole different story than killing some Nazis and going home afterwards.