For me, if I want to take a planet like that very early, then I skip the econ techs and go straight for the bombers and maybe even some bomber starships. Load those into a transport, get some chaff ships around to take the fire off the bombers (fighters, etc), and maybe throw in some light starships to buff the bombers. Fly in, unload the transports in a manner that lets me get at the fortress without my bombers getting shot TOO much, and then ream it. Then go home, focus on econ, and come back and trash the rest of the stuff.
Alternatively, sending multiple waves of ships to take out a guard post at a time and then getting the fortress once I've built up some. It's not as economically efficient, but it can be a viable way to take a planet like that while doing something else.
Or: more often than not, these days I tend to just ignore those planets until later. I really like getting early econ techs as well, and so I'll do that, then take a few easy planets, and suddenly I have a rolling economy as well as a bunch of extra knowledge. Then I come back and put the smack down, and having both higher-tech bombers AND multiple waves is fairly trivial. I like to take 2-3 planets in the first 45 minutes, so then the bigger planets like that in my hinterland would be what I'd come back for next.
Welcome to the forums, by the way!