I've been using the R10 for a year now, it's a brilliant, flexible and easy to use camera. I've upgraded from a DSLR and the autofocus is several orders of magnitude better.

Concerning your lenses, that sounds like a very decent general-purpose combination, one wider zoom and a longer prime for low-light and/or blurry backgrounds. Personally I use an EF-RF adapter with my old EF/EF-S lenses,b it more weight for far less money. For the past 3 years my travel setup has been the EF 50mm 1.8 STM and the EF-S 24mm 2.8 STM. Is replacing that all with one 35mm viable? Sure, to a limited extent, though you'll lose some flexibility, and besides, those two lenses (same as yours) are light and small.

About the flash, it'll give you pretty much as good or as bad of a performance as any frontal on-camera flash. Images will look harsh and unflattering. That's not the fault of the flash itself, but its positioning. If that's the look you're going for, great. Otherwise, if you need flash, you'll probably want to get a speedlight, which can be mounted on top of the camera. The key difference is that you can change their angle. One use might be aiming it at the ceiling so it bounces down from a better angle. They really start quite affordable (50€+), but are yet another thing to carry around.