> Securing the borders from what?
Russia has been invaded by land countless times over the centuries as it has few natural barriers. Securing good natural borders has been a major national security concern for Russia for centuries and up until the fall of Soviet Union they had secured all of them. They have been winning those back gradually over the last couple of decades.
> If he was worried about NATO he would be beefing up troop deployments along the borders of Lithuania, Latvia, and Estonia where he has withdrawn troops
The Baltic states are likely future targets. Thankfully they've been doing so badly in Ukraine they are being forced to withdraw forces from other areas and debasing their ability to achieve those future aims.
> They have nuclear weapons and would have a justifiable reason in using them if they were wantonly invaded
Firstly, there are numerous scenarios where Russia is not wantonly invaded but perhaps different powers use salami tactics either directly or by proxy that wouldn't justify nukes at any particular point. Secondly, Russia may not want to rely on its ability to maintain nuclear weapons long-term especially while its industrial base decays, the result of its crumbling education system unravels, and its demographics rapidly decline. Regardless, it's much better strategically to have good defensive geography and nukes.
> It seems so many want to run away from the straightforward situation...Everything else is a sideshow at best, and Russian propaganda at worst.
The idea that Russia wants a secure border geography and that Ukraine is a barrier to that is not new nor controversial. The (continued) invasion of Ukraine was long expected and the high level reasons long known.[1][2]
1. https://www.theatlantic.com/international/archive/2015/10/ru...
2. https://zeihan.com/russias-twilight-war/