What do you mean by solid border?
All Ukraine needs is to control NATO agreed borders.
Also consider how this war will end. It’s not going to grind into a resistance, Russia will withdraw troops…probably without any kind of peace settlement, they’ll just declare the special operation over and go back to the decades old borders. That’ll happen either because Putin caves to domestic pressure or there has been a coup.
So who is contesting the borders? The Russian backed militias that fought 2014-2018 in Donbas and Luhansk stand no chance against the nato armed Ukraine of today and the population have turned against the “independence” movement.
Crimea is sorting itself out with all the Russians who moved there over the last 8 years fleeing across the bridge now they’re in HIMARS range.