Do you ever get riled up over a situation you imagined completely out of your head?

Constantly. My internal monologue is pretty much just multiple versions of myself arguing different points of views on things. Sometimes shit gets heated.

Just as Mark Twain said

I've had a lot of worries in my life, most of which never happened.

Would it make you more or less frustrated if you imagined you were gym staff trying to explain to someone that they can’t cancel because they never had a membership?

This is a classic symptom of anxiety. If it begins happening regularly you should consult someone.

I noticed this basically stopped happening once I made an effort to erradicate all ragebait content from my internet diet

Yes too often and I find it can sometimes leave me emotionally on edge as if it really happened. Wish I could just turn it off.

I practice trying to talk a cop out of a ticket, ordering food at a drive thru and then asking for the food to be corrected, explaining why I used the wrong door at work, declining a happy hour invite, etc. Then sometimes later i have to work hard to remember which ones were fake. I catch myself starting to tell a story about something that happened today that didn't really happen.

I have full on, rather extravagant rants in my dumb little brain hole on the regular. They usually start out as one sided conversations regarding some imaginary situation.

Yup, sometimes when I'm dealing with something in real life I imagine handling the conflict in different ridiculous ways. Usually irl it ends up being handled smoothly, but on the rare occasion that the beginning of one of my imagined scenarios come true, I always end up getting a huge spike of anxiety that makes me try to get out of the situation as quickly as possible instead of doing things the way I had imagined

No I am too busy kicking myself for things I haven't done irl to worry about made up shit.

Man, I'm lucky I don't end up punching myself for the shit imaginary things in my head say.

Consider yourself lucky that you don't have enough real problems if your mind has to invent imaginary ones.