What would cause my rear derailleur to be so clunky?

Sorry for the bad title; I’m relatively new to wrenching on my own bike (due to my LBS’ impending retirement, I’m trying to learn to maintain my own bike so I don’t have to pay Trek store prices) so I don’t know specific names of the parts I’m talking about.

My bike was riding fine until I got a flat on my trainer last Tuesday. I took the wheel off and replaced the tube, which went well.

But ever since I put the wheel back on, I’ve been having the issue you can kind of see in the video.

The little middle cog (is that a pulley wheel? I’ve seen that term thrown around) seems to be sticking, or maybe my chain is sticking, and then it’ll jump a couple links all at once. It feels _okay_ to ride, but definitely not great, and I seem to have lost the low end in the rear. If I shift down to the smaller cogs, it rides super rough and will even upshift on its own.

I’ve done some work adjusting my front derailleur after a crash and that was pretty simple, but there are so many screws to adjust on the rear that I’m nervous to touch it