Hi Daniel,
Over the last 10 years I was about 50 times in the former USSR. Therefore, I think I could give you some advice. I would not go for suggestion 2. Reason is Belarus. You are offering a climb in this country that is not cycling-friendly at all. Accomodations are scarce and safety far from garanteed.
Suggestion 4 is a very nice option in order to cycle through Russia. I've done many kilomters over there and it was always very pleasant.
Kind regards.
Nico