If you are only interested in the stuff that ties into the DS9R, then you'll probably be OK with just the DS9R books. I don't think there was really anything in The Soul Key that you would need to read RLL to see the end of. RLL does follow on from what happens in TSK, but it was more important to the other MU books than to anything specific to TSK.
So the DS9r pretty much ends at The Soul Key then...
I keep seeing people say that. Actually there was another book printed afterwards called "The Never-Ending Sacrifice", and THAT appears to be where the DS9 relaunch ends. Well, so far anyway. I'm in the middle of catching up right now with Warpath, so I only know things up to that point. But, the book I cited above is the last title in publishing order.