Anyone found good free SOPD study resources besides the obvious ones?

I've already gone through the standard "SOPD" results on Google and most of it is just selling prep courses. Looking for actual free resources.

What I've tried:
- Practice tests here (solid, especially for SOPD - System Operator/Power Dispatching)
- A few YouTube channels but the quality is all over the place
- Reddit threads from 2+ years ago (some outdated)

What I haven't tried yet:
- The official SOPD study guide — is it actually worth reading cover to cover?
- Library resources — anyone actually found useful materials there?
- Specific YouTube channels that cover SOPD exam well

I don't mind paying for something that's genuinely better than free, but I want to max out free options first. Budget is tight.

What resources did you use that you'd actually recommend?