Hello Axmo, you're right to write.
I'm a PwP, not a family member. But I observe how my very mild symptoms affect my o/h from time to time - and I'd say no, you're not over-reacting. At least we PwP have got our symptoms to focus on: our carers and family can feel less in control.
It must be hard, too, being the only one let into the "secret". My o/h and I haven't told anyone either and I'm cautioned by your story to be careful who I place that knowledge on.
Parkinson's is usually slow moving, doesn't always involve dementia, and drugs are mostly a real boon. All positives to latch onto. I'd say your dad needs you now . . . and I hope we on the forum can be the help you need in turn.
More practically - does your dad have a PD nurse? They're a boon, and maybe you can ride along to your dad's next appointment. In my experience they welcome informed and helpful family members.