Yeah, BSG is really not a good format for spin offs.
Pretty much. The only spinoff idea worth anything was Caprica (in part because it originated as an idea for an original
series), and we saw how that turned out.
Ron Moore's Battlestar Galactica, like Babylon 5, Firefly and Farscape before it, is not well suited to franchising in the way Star Trek is - that it closed its big conceptual loop and answered the probing questions of its universe could leave any prequel series - and it'd almost certainly have to be prequel - feeling a bit redundant.
It's best to just wait for the next thing. Maybe Defiance (which has a fair number of ex-BSG writers and Bear McCreary) could become that. Or some other show.
I think you are largely right when it comes to the re-imagined BSG and Farscape (thanks the Peacekeeper Wars mini), but I think Babylon 5 and Firefly have a lot of franchise potential.
With Babylon 5, even though the Babylon stations are finite, JMS created an interesting universe around the Babylon stations to play with. Granted Crusade, Legend of the Rangers and his untold tales series didn't take off but I chalk up Crusade's failure at least to network interference. I think there's a rich past, present (running alongside the Babylon 5 series), and future that could be mined in that universe.
As for Firefly since the Alliance's ultimate fate is still unknown, I think there's still life left in Firefly. Plus we never saw much from the Alliance perspective. And there's also the war they could always go back to and dramatize.