He should have just done this instead of invading, imho. Much more cost effective, like how he's propping that guy in Belarus. Just orchestrate a coup to get rid of Zelinsky and help install a more friendly president.
Perhaps, but neither strategy is easy. The President before Zelenskky was a pro-Russia type leader that was essentially ousted who now lives in exile. Whatever coup Putin could devise would probably be shut down by the West before it could come to fruition. (Though seeing as NATO has done little to intervene in the invasion, maybe I could be wrong about that).
The invasion stirred nationalist sentiments in Ukraine
I agree, but Putin did not foresee this happening. He envisioned a quick Ukrainian defeat. Instead he's galvanized Ukraine and the rest of the Westernized world against him.