Tool is showing 0.75 on an ultegra 11-speed chain. I rode the bike 5k miles. I just got a new chain. Haven't put it on yet...

Question is : Did I already ruin my cassette, front rings and jockey wheels?

You probably did not "ruin" the rest of the drivetrain. Shifting might suffer slightly.

I've used this very general guideline: replace the cassette every 2-3 chains and replace the chainrings every 2-3 cassettes. YMMV (a lot).

Unless the jockey wheels are excessively worn or shark-toothed they will be fine - and I can't imagine that level of wear occurred with just a single 5000 mile chain.

This reminds me that I need to change out the cassette on my road bike soon

Not likely

Thanks guys! I was sweating bullets.

Chainrings and jockey wheels can take a lot of abuse. I wouldn't worry about them. Throw the new chain on and if you get skipping on any of the cogs then replace the cassette.

^^^ What he said. While component life varies widely with the quality of what you buy, how well you maintain it, your riding conditions, and your power output, this guidance fits my experience as well. But for some people their choices and conditions mean new chains much more often than 5K miles and therefore these rules are meaningless. YMMV (a lot).

HMJ gets a bit more mileage than I do, but IME two chains per cassette and 3 or usually 4 chains per chainring. Most of that was
on 8-9 spd cassettes. Wide range 11-12 spd cassettes may result in more wear in the 11-28 tooth range than the 30-40+ tooth
area especially with road riding. Depending on your area compact/subcompact CR tend to wear mostly on the larger CR. Helps
to have a chainset with inexpensive easily obtainable CR. Jockey wheels take an enormous amount of abuse before wearing out.