Russia isn't going into Romania or Bulgaria. Once they hit the border, it only becomes a question of timing for when they grab the Russian controlled bit of Moldova though.
Russia invading a NATO country is just as much a redline at the moment as NATO deploying military forces into Ukraine/against Russia.
If Russia is prepared to eat the economic & diplomatic damage, they will be allowed to grab off Ukraine and that bit of Moldova (and no, Moldova won't be allowed into NATO before that). But Russia won't be allowed to go further.
While Russia would like to also grab off Lithuania, Latvia, and Estonia; they probably get bogged down enough in Ukraine to not make the attempt.
If Russia moves past Moldova towards Romania or Bulgaria, Turkey will move North to defend them even if no one else will. Turkey very much does not want to share a border with Russia.
The Baltic states aren't going to be allowed into NATO, but they absolutely will be given trainers and military equipment sufficient to make a future Russian attack into another quagmire.