Russia needs to quit while they're ahead. Trump wants to be a peacemaker and will likely give Russia a favorable off-ramp. Quitting sooner gives them a chance to draw defendable borders while returning bad land in return for easing of sanctions. It's stealing someone's lunch, eating the best parts, and selling the scraps back.
Holding up the process could put NATO in a position where they feel they must force Russia to the table by further increasing sanctions, unshackling what Ukraine can do with the equipment, and having pro-NATO mercenaries suddenly appear to help Ukraine. Russia's war economy has been resilient but it won't last forever and China isn't going to prop up Russia if they become a liability in the long-term.