Do we know for sure that JJ had no plan with what to do with the ST?
Based on the things Rian has said, it was entirely up to him on what happens to Luke next. He has said, I believe, that he basically just picked it up from exactly where JJ left it off and made it his own thing. I’m sure there are greater discussions with Kennedy et al. that they’re not going to tell us about for spoiler related reasons. But from everything JJ has said, his decision to leave Luke was due to what he wanted to get out that film as opposed to a desire to set up Luke’s role in the trilogy.
We will probably learn more once the ST can be discussed retrospectively.
Edit:
Or that it was up to him, for that matter.
We only really know what we’ve been told. Both JJ and Rian have expressed that they had a tremendous amount of freedom with regards to what they do in VII and VIII. Of course, we assume there are limitations and restrictions, and that they aren’t going to discuss things that were forced upon them. But decisions involving Luke have been made by JJ (leave him out of a film) and Rian (something unexpected that apparently Mark Hamill initially disliked).
So, my assessment so far is that of course they don’t have complete autonomy, but they have more than I would have expected.