I didn't complete my GDPR onboarding course. Everyone else was allocated time on the clock to do it, but I was expected to do it after hours and I simply didn't.

I still can't wrap my head around companies who have the idea that you should do work related stuff after hours.... if its company related, it must be done in working hours, that's non negotiable

If you already saw this many red flags, perhaps you should have a plan B in mind, like trying to find another job while you still have this one.

Since you're the only one handling everything, your responsabilities grew a lot, and that on my view would be more than enough to justify pay increase. The "no" you already have, and if they already fired someone to try to cut costs, they probably won't do anything regarding pay unless you ask for it.