> Since Independent Scotland is very likely to rejoin EU

That is in no way clear because Spain has previously (during the Scottish independence referendum) indicated they would veto such a thing (because they don't want to give Catalonia any ideas or hopes).

But in any case, a Scottish independence would force the UK to find, and invest a lot in, a new base for its nuclear submarines. Scotland is also the most important UK part of GIUK gap, where there is a lot of infrastructure to catch and track Russian assets entering the North Atlantic. UK losing access to that is not good for it nor for the US with their special relationship.

A Scotland in the EU can also create tensions between the UK and the EU, which would also be good for Russia.