Realistic all the way for me; it looks more various to me when I'm setting up a game. That said I should really give maze and circles a try. Would someone mind telling me what the difference would be between clusters and the current hubs setup? I'm not sure what you mean by it.
But to answer the question, I've never had the patience to finish an FS game but I have played two to solid midgame states (2 & 4 cities, respectively) on 80-planet realistic maps (~7.6 diff, spirecraft medium and no golems or hybrids) and I don't think it's that hard, although I can't cover all the ways into my core with cities and thus wind up relying a lot more on the spire fleet. It's pretty easy to manage the general direction that exo-waves come from with gate raiding, so I wound up with basically two fronts in the game I played farther into. One was only two planets wide, and one connected only to a long chain, so I got the AI to ignore it by neutering the three or four worlds of the chain (it's possible the AI should be modified so this trick doesn't work) and funneled everything into a huge fortress world. That actually didn't have a city, as I wasn't sure the backdoor trick would work when I build my first city, but it had two fortresses and a ton of spirecraft siege towers, attritioners, and implosion artillery, as well as a few martyrs for CPAs. Then the other side had a less convenient setup where I had three border worlds all connected into a little clump, and I was able to hold that with more modest emplacements, two cities, and a quarter to a third of my spire fleet on defense.
Anyway, the end result was a very defensive game and a lot of rebuilding outposts outside my core, but the spire fleet was still massively powerful enough that I could often transport it out and take a couple of worlds or fetch a new shard between exo-waves. I'd estimate this was possible every other or every third exo-wave; depending on what direction the attacks came from I'd either come out pretty much unscathed or lose enough to force me to use the whole calm period rebuilding. But I think I could probably take out the AI from where I am in another two, three hours without going all the way to the capital, or probably finish that way with more time.
That said, it was a hell of a slog --- how long is a game normally on a more FS-friendly map type? I got so tired of the length of that game I've been playing exclusively 20-30 planet maps since then.