There are lots of strategic and economic reasons (prima facie) why the eastern parts of Ukraine (including Crimea) if not the entire nation are valuable to Russia, given how fertile the land is and how much Black Sea coastline there is.
As it’s worked out, at least as things stand, it’s ended up costing Russia far more than it could hope to gain, but as you say Putin wasn’t to know that when he invaded, following which he inevitably doubled down.
Even if he retains what he keeps he’s either going to end up with sanctions continuing (and the effects of that aren’t just economic) and/or what remains of Ukraine in NATO (or being afforded protection by NATO) along with Finland and Norway. Either way, Russia’s national standing has been hugely reduced, possibly irreversibly.
There is no way any sensible person could paint any of that as a win for Putin. He’s hugely fucked up, by any objective measure.