For a start Russia already has a Baltic state: https://en.wikipedia.org/wiki/Kaliningrad_Oblast and, not only that, has access to the Baltic itself. So there's zero geopolitical strategic advantage.

Anywhere that Russia has "annexed" has had a majority ethnic Russian population and historical ties to Russia. Estonia, Latvia, and Lithuania are not majority ethnic Russian, do not speak Russian, and are part of the EU (and NATO) so they're clearly not aligned geopolitically with Russia and it would cause major regional ramifications, undoubtedly triggering a war with NATO, and/or the EU (both nuclear entities). It would be wholly unprecedented in modern times.