Running a Gelatinous Cube encounter

You are right, there's only so many ways that you can say "you hit the cube and it jiggles." It would be a pretty boring fight if that's all that happens as the party whittles down its hp. My philosophy with issues like that, is if I can't make the fight exciting or interesting with what I'm using, I need to change what I'm using. Single monster fights are already not easy to make interesting or fun so it's honestly not a bad idea to generally avoid using them. Adding more varied enemies create more need to strategize. Two cubes is more interesting than one. One cube and a mage is more interesting than two cubes.

But if you still want to use a single enemy (for things such as fighting that cube in a hallway that it would fill, which isn't a bad idea that you had), you should look into making the enemy more interesting itself. Some homebrew mechanics thrown in. For example, what happens if your barbarian cleaves the cube vertically with a battleaxe? Does it make two smaller half cubes, or maybe two rectangular jellies? What happens to it's hit points and ability scores? What happens if they slice those smaller cubes? Can the cubes re-convene and possibly heal or redistribute their hp? Maybe if they're small enough and get hit by a fireball they just fizzle away because they're too small to resist being dried up.

I know that I didn't technically answer your question about how to describe it, so let me try that too. A biting companion may bite part of it off and you see some of the bits slide down the outside of its mouth, the missing chunk is filled in by more of the cube, but you see it become ever so slightly smaller (maybe if there's light on the other side they see light peeking around the edges). You should try and think of what actual effects would happen to this thing to describe them. Like maybe a fireball burns up a chunk so those bits either get pulled further into the cube to protect them or maybe they get ashy and burnt and crumble off.

The way I make my combat flavor text is by thinking of the effect of whats happening, and explaining it. Which sounds simple but many people don't think of it that way and just describe a hit followed by the defender recoiling. It's hard when you just use one single stock monster from the monster manual, but if you populate the battlefield with several different creatures (that make sense to be fighting together), you'll be able to describe a large amount of reactions because the things getting hit are differently. If you still want to use one monster (which there are many good reasons to do just that), then make interesting effects for what happens to the monster to inspire more flavor text.