Those are all excellent questions, and unfortunately I don’t know all the answers. I’ll try to answer what I can despite their loaded nature, and say when I don’t know.
There are a few different commonalities; they generally identify as slavs (with exceptions, especially in the middle east like Kazakhstan), most former USSR states are member states of the CIS, almost all nations have people within them that identify as ethnic Russian, and naturally most share borders with Russia.
That’s a good point, most former Soviet states don’t have a modernized military and likely wouldn’t be capable of withstanding a Russian invasion, however I think it’s worth pointing out that the majority of the West also thought Ukraine would fall during the initial invasion. Generally, I don’t think I understand your point here and I’m genuinely interested in the reasons you brought this up.
Honestly, I have no idea why the northern Slavic nations haven’t seen more aggression from Russia. It’s possible that the Kremlin doesn’t see them as valuable, though they have seen some disinformation campaigns and political propaganda via proximity to Russia and Belarus.
I don’t think NATO has been a deterrent, but it’s possible that I’m wrong. I think it’s worth pointing out that an excuse for Russia invading Ukraine was explicitly NATO trying to expand into Ukraine. They didn’t have much interest in doing so either, until after the initial invasion saw Ukraine still standing.
Those are all excellent questions, and unfortunately I don’t know all the answers. I’ll try to answer what I can despite their loaded nature, and say when I don’t know.