IMO the only regime change likely in Russia will not make you happy. There're some very pro-military leaders arise recently. Putin does not allow for war to go full-scale. Those leaders will.

Fragmentation is possible but I don't believe it, at least for now.

Russia loses soldiers but they obtain much more soldiers with perfect battle training, they acquire priceless experience as to which weapons are effective and so on.

It's not that simple.

If anything, threat to eastern Europe is as huge as ever.

Sometimes I think that Putin slows this war on purpose. To train more soldiers, to weed out bad blood from commanders, to put their weapon to real test. Basically to build a real army.