Am I alone in writing libraries in my time off work with the intent to use that library at work to avoid my company owning that library?

I work for a university. I'm 90% sure they can claim any work even if completed off hours.. actually, maybe not, I didn't send in the signed copy of the agreement, but I'm sure they'd push it if they could

Nope.
I do the same thing.

I also refuse to sign anything that claims ownership over anything I make in my own time (or ability to show off work I've done for them, even if it's just pieces). I fight tooth and nail for those, and if they refuse, I walk away from the job offer.

Same here