There are other options to annexation though. Keep fighting until the Ukrainian government agrees to an unconditional surrender. Then impose the following:
1. Ukrainian acceptance of full responsibly for the start of the war with or without reparations.
2. A partition of the country with Russian speaking portion of Ukraine either becoming independent or integrating into Russia. Russia will decide where the border will be.
3. No airforce, no navy, no air defence. A numerically limited army with no tracked vehicles. No NATO.
4. Denazification. Prosecution of Nazi war criminals under Russian law, removal of statues, and renaming streets glorifying WW2 nazi collaborators. Make display of Nazi insignia a criminal offence like it is in Germany today.
5. Dismantling of all nuclear powerplants. A no nuclear policy in the west, with no foreign biolabs.
6. After all of that is complied with a full Russian withdrawal, free and fair democratic elections with UN observers with no Russian interference. Even Zelensky could come back from America or Israel and run the country again if he wanted to.
7. Russia reserves the right to intervene if any of the above are broken.
All of the above would be perfectly legal according to international law and would have precedent - it's a mixture of the outcomes Germany was given by allied forces after WW 1 & 2. It's actually more lenient as there still are American bases in Germany to this day. There would have been Russian bases too if Gorbachev didn't withdraw them.
There's a few advantages to this over just annexing the whole country. First it means Russia doesn't have to pay for rebuilding of all of Ukraine, let America do that. Secondly as the dispute was resolved bilaterally, there would be no justification for international sanctions. Finally it also means Russia doesn't need to deal with the self hating wannabe American Ukrainians, why would anyone want to share a country with them? I thought pro-colonial HKers were bad, but they have nothing on the Ukrainians.