“What would be the point of Russia fighting on at all if not to take more of Ukraine territory?”
Russia’s current goal is the destruction of Ukraine’s military forces. Taking and giving land just provides the playing field. Ukraine’s military is famous for allowing its troops to be encircled and mauled and Russia loves that. The idea is for the other side to run out of sufficient soldiers.
This has nothing to do with the fighting ability and bravery of Ukraine’s soldiers, which is very high. Russia is mobile, Ukraine is in fixed positions. Very old situation and the outcomes are well established. Defense loses. Once the army is defeated, you take whatever land you want, if you want.
Once that is accomplished, if you want land, you just march in, put up your flags and start your governance with military occupation.
Putin’s stated goal is the genocidal “destruction” of the Ukrainian national identity and culture outside of the filth of the Russkiy Mir. It doesn’t get any more “Nazi” than that.
If you call having to walk 20 plus miles from your bunker back to the supply depot to get water and food "mobile" then perhaps the Russian soldiers are "mobile". The Russians in Ukraine along with the Ukrainian defenders are only able to survive if they are in bunkers, basements, or trenches with cover. That situation favors the defense.
Take a look at how Russians move about near the front lines.
The road from Zhovte to Novopustynka south of Pokrovsk. Littered with burned Russian vehicles, not many things with wheels are left.