I wouldn't call the Baltic states "the most heavily fortified border of NATO", even according to the NATO analysis they will not be able to resist a full-scale Russian invasion for more than several days. The goal of the forces deployed there is just to buy time for consolidation of forces deployed in the rest of Europe for counter-attack before the states get fully occupied.

Balkans also have a little of value for Russia in the current state. A more interesting target would be to take control over Georgia or Azerbaijan (not necessary using military means), to establish a better control over oil and gas routes from the Middle Asia.

But a more realistic, much more valuable and easier target is the south-east Ukraine. If degradation of the Ukrainian state institutions continues with the current speed and the ongoing cold civil war will not be cooled down by a more sensible cultural and language policies, I will not be surprised to see in 5-15 years a full collapse of the state, significant social and economic instability, with breakaway regions following the Yugoslavia scenario, some of which will fall under the Russian "protectorate" for gradual later integration. We already can see early signs of it in the results of municipal polls, weak central government, and totally unprofessional behavior of the "elites" (who in the *#!? right mind spends most of money from the COVID fund on road construction and other non-medical goals?! who launches a full-blown constitutional crisis using clearly unconstitutional means instead of slightly tweaking the law in question or trying to engineer a constitutional reform to restart the corrupted constitutional court? who pays a ton of money to "anti-corruption" departments which after working several years haven't return a dime to the budget? who effectively strangles small businesses when they are already hit by the pandemic? etc. and etc.).