One option for Putin is to withdraw all his forces to the Donbas
Withdraw what?
The BTGs in Kharkiv have abandoned their equipment to escape being overrun, the 25 BTGs in Kherson are trapped and would have to do the same.
They’ve lost 50K KIA which means around another 10-150K WIA/MIA and the Russian land forces started out with about 350K including reservists to defend the entire country.
In summary even in the concentrate their forces they are short of men, and they now don’t have the equipment to supply them.
so, if they do pull back - what to they do as the Ukrainians keep coming?
Stopping a retreat and equipping, reorganising, get your logistics sorted out and supplies reestablished takes time - and father winter is coming with the snows in 8 weeks or so.