“So expansionism is very much what Occams razor leads us to conclude.”
Only if looking at the last five minutes.
Are you seriously suggesting that Putin went into Ukraine with a tiny force in order to advance Russian expansionism?
If so, you should get help.
-fJRoberts-
Yes, he did. He intended to overthrow the Ukrainian government and put in his puppets, a la Belarus, in a coup de main.
Frustrated in that, his war aims have mutated several times apparently, as their various attempts at overcoming the Ukrainian resistance have sequentially failed.
Now he wants to eke out a win by preserving his remaining territorial gains.