The problem I see they have with Transnistra is if they create waves, Ukraine and Moldova can simply close the borders to everything going in, being landlocked they would rapidly run out of food, though not sure who provides water and power though, I would assume the officers are rotated through Moldova, that could also be stopped, And there is no way Russia is going to be able to resupply from the air, Ukraine could see to that. I would also think Romania would step in to aid Moldova, bearing in mind they are intertwined. I would also imagine Ukraine could use the weaponry stored in Transnitria too.
As for Russia's next invasion of the Baltics etc... The question has to be how and with what? As things stand at the moment they are just about maintaining supplies to Ukraine, if they God forbid take Ukraine, it will not be over, the population as I see it will fight them tooth and nail with the equivalent of the French Resistance.
This will mean Russia will need to deploy hundreds of thousands of troops and equipment just to try to for some form of stability, unless the mass genocide of the Ukrainian people is their aim.
So where are all these extra troops and equipment coming from, if they had them they would be using them?
Do they pull them from borders elsewhere to bolster the troops to move into the Nato countries in the Baltic states, if they did what would there be to stop the likes of Finland opening up a second front and how would the Russians be able to cope with fighting on two fronts etc? Throw Transnistra and Kaliningrad into the pot and anything could be possible..