Fortresses as special forces is an awesome idea, and as long as they can planet-hop totally fits what I was looking for.
Plus there's a lot you could do to expand that idea in interesting ways for defensive choices - maybe a later expansion could add different kinds of fortresses (this one is faster but lightly armored, this one deploys droneships, etc). If mobile forts are meant to be a major part of a player's defensive terrain in addition to an emergency reserve, it might even make sense to have a special "fortress cap" and things that affect that cap.
Anyway, that's all theorycrafting, but I love the basic idea.
I also like the idea of the SETSSOF (I think you are stuck with that acronym now
). More ways of adding terrain to the galaxy map and forcing hard choices is good IMO.