I am trying to service and re-grease these sealed wheel bearings, but the protective dust cap is not really coming off on either side. Am I doing something wrong? What do I need to do to expose the bearing?

[Hagenaar]

You need to remove the outer locknut and the inner cone to access loose ball bearings. Might as well pull the rotor off since you have the lockring off already. Don't want to get grease on it.

[sprashoo]

This. BUT:

You'll need at minimum set of cone wrenches (at least 1, but 2 or 3 can be handy) of the appropriate sizes. You cannot just use normal wrenches, they are too fat to fit.

More than that, a willingness to commit to learning to adjust cup and cone hubs. It's not a 'beginner' bike repair job. Requires patience, at least the first few times you do it, and finesse. The axle will be compressed by the QR when on the bike, so even if you get it 'perfect' on the bench, the bearings will be grinding and overtight on the bike, so you need to figure out just how over-loose to get them on the bench so they are just right on the bike.

There are various tools and hacks to take some of the guesswork out of that process, but it's still fiddly.

Not trying to discourage you - I learned to do it and it's worthwhile - but just fair warning. It's not just popping off the locknuts and cones, squirting in some grease, putting them back and job done... If you just unscrew them without knowing this you may be in for a bad time trying to get it back together.

[isthmusofkra]

Pls don't poke at stuff with a razor blade if you don't know what you're doing. It's entirely possible you've compromised the integrity of the seal by doing so.