Ukraine as buffer
A colleague, who had grown up and been educated (very well as a PhD scientist) in E Germany, told me Russia viewed Ukrsine and many former satellites as a buffer against invasion (by Germany).
Whilst he accepted Europe invading Russia a third time would never happen, the fear is deeply ingrained in the Russian psyche.
So ex-satellites joining NATO only increases that imagined fear. He asked how the US would feel if Russia ignored the Monroe doctrine, built up the militaries of all the Central American states, including Mexico. The Cuban missile crisis notwithstanding, I think the point is a good one. If anything, Cuba demonstrated how sensitive the US was to neighbors undet Soviet influence.
If the Ukraine stops begging to join NATO or have a NATO build-up in response to the Russian build-up, it's possible the Russian forces would go home.
Of course, I am no expert and trusting Russia may be naive, but his logic did make sense, to me at least.