From talking to a Marine in my class in NFO Advanced it doesn't appear like there is any standardized path that they are giving WSOs, at least not that they have told him about. Unfortunately he has a weird eye issue that is a DQ for SNA otherwise he would have switched a long time ago. I think part of the problem is that the date for the sundown of the -18D keeps moving around which makes it hard to say where he will be in his career once that decision needs to be made. I think his personal preference would be to lat xfer into the Navy and be a Super Hornet WSO, but I don't have a clue how realistic of an option that would be.
At the end of 2018 the USMC reassigned most of their SNFOs to SNA. There were a few that were already past API that were medically unable to switch for one reason or another and now they are the last batch heading through flight school at TW-6. I think there are 7 or 8 total at various points in the pipeline. The last one will probably wing late next year.