Never ever risk your stability and complete control of your bike in going downhill, much less take one hand off the handlebar even for a second when you may need to brake strongly on a steep slope. It's a recipe for disaster even for highly experienced bikers. You can't beat physics, this is all about vehicle dynamics and you want to stay safe.
If you lose stability and drop to the ground or even just go swerving uncontrolledly downhill with heavy traffic behind you, that's the closest you can avoidably get to "this is it" in an up-to-there normal everyday traffic situation. Cars and heavy vehicles can't beat physics either - braking is much less effective downhill. Better not count on a vehicle behind you being able to conduct serious braking at all.
So your first priority is to keep your ability to go straight and follow the road (or to get off the road in a controlled and timely way if that's preferable), along with your ability to completely control your speed and not find yourself off the exact place and trajectory on the road where you wanted to go in the first place.
If that means you can't signal at any given moment, because that means taking one hand off your bike, then you can't signal at that moment. When that's not an option, so is getting into that situation where you have to fill too many requirements simultaneously.
Fully taking the lane is a good idea where it helps, in a way where cars behind you can easily control their own driving (no imposing surprises on anyone), if it makes truly impossible for cars behind you to do things that can turn out risky for yourself, such as overtaking narrowly.
That's not being rude when it's what it takes to stay safe for all parties involved. And you have the same right to be there as every other road user (provided you are keeping to the regulations). If it is rude in a given situation, then there's something different wrong already. By doing it properly, you are not just protecting yourself, you are also protecting the respective driver from prosecution in an accident that could arise.
If a particular place or situation is such that you're out of these options when you go there, you probably don't want to be riding in that place or situation. Protect yourself and use whatever other option is available. You want to feel fully confident with what you're doing and with all involved persons' complete control of the situation.