Agreed. Minimum should be Eastern Ukraine but I would add to that, all the coastal areas in order to get a direct land border access with Romania and Moldova. Russia isn't interested in Romania, but it definetely is on MoldovaRussia might not be able to diplomatically push for this. They've always had to give something up.
This is a minimum of course. Russia won't settle for LESS than all of Eastern Ukraine.
Moldova can become the next Ukraine situation so it is important to have access to it
For Western Ukraine, Russia's maximalist demand would be demilitarization and non-alignment/neutral status. Depending on Russia's "position of strength" at the time of negotiations it can lower its demands