It felt like the calculations for relative strength we're really strange with Muscovy's vassals, but I guess it worked out in the end. Had to develop some of their grassland provinces a bit to get them happy early on (wanted to avoid anyone supporting them), past that it was going smoothly. Was actually surprised how easy it was in the end.
Having a tough time finding decent allies though, almost had Poland at one point but then they went and tried to claim my throne and then broke the royal marriage when that didn't work out. Working on Austria now, but no success so far. Actually considering supporting Sweden now, but that probably wouldn't be long-lived either...
nice to see it worked out. I would NEVER integrate them, in all honesty.
as for allies: as soon as leagues form in the HRE, join the side who has the most greater powers that aren't your rivals, and ally all of them. joini the league and all the nations in it go friendly status. then leave league again after 5 years.
supporting sweden isn't a bad idea at all either