Sacred Haven of Debauchery

Bombardments completely halt production. There's no more rutabaga, it's all gone...

Since you're arguing from a realistic standpoint, two corvettes won't completely block production. And even if a large fleet is doing this, it still can't establish control over the planet; it can only wait for everyone to die of starvation. Meanwhile, the existing army will be shooting deserters and panicking surrenderers on the spot. So, in principle, everything is correct: the fleet can hang around in the air as much as it wants, but without ground control, its usefulness will be limited.

So, how is it?

Well, if there were a couple of tens of millions more of them, they might have won.

Actually, yes, I also don't quite understand what all these difficulties are caused by. In the initial version, there were almost no armies on the planets at all; just three armies of xenomorphs were enough to conquer what was needed. Now, you need more, especially against the fallen, but as Ple-Sen rightly pointed out, ten elite armies with upgrades are more than enough.

if a large fleet is doing this

Yes, I was just thinking - how large would a fleet have to be to completely make it impossible to grow rutabaga even on a small planet like Earth? In general, if we go further in realism, it's likely that such a policy won't work; they won't be able to take anyone by attrition.