Suitability of Giant Aero RS2 for occaisonal on-road mountain trips I'm planning to buy a bike for use only in summer, mostly on 100% flat terrain (being The Netherlands). However, I would also like to be able to use it as a packing mule and go into the mountains for a week, on-road, once a year.

If you take Giant, they have the Expedition LT for the latter. But, it does come with a hefty price tag (€ 1900). Giant also has the Aero RS1 (€ 1300) and RS2 (€ 900). The RS1 having a completely different frame, being lighter (which is not relevant for me, since 1 kg on 30 kg of bags won't make a difference).

I know that the Expedition has better Shimano sets, hydraulic disc breaks (although I have my doubts about disc breaks; spoke shearing forces, complexity), etc, but I was just wondering how a bike like the Aero RS2 holds up when going into the mountains, on-road, once a year for a week. It does have mounting holes for front bag carriers, so at least in that sense, it's prepared for it,  but they do call it a 'lite trekking' bike. I just wonder if that means something, or if that is only to give credence to the existence of higher priced ones.

The only thing I may actually miss, is the hydraulic breaks, for their better control. Giant has other bikes too that have hydraulic breaks, like the Futuro RS1, but its front fork has no mounting holes for bag carriers, and it has suspension (which I don't like climbing).
If you're going on road, a hybrid should be fine (like the Aero RS2) - the gearing and tire size and handling should be reasonable provided you don't go too high on the weight. I'd probably not load it too much though (for that you want something intended for touring). At 30 kg of stuff though, I think I'd look for something more touring bike-y than the Aero RS2 - its part of Giant's casual series and carrying 30 kg of stuff isn't really in the casual realm and you could likely do better for 900 euros in that realm. 

A more reasonable option overall would be to buy a bike that works well outside the week of mountain touring, and rent a bike for the week long mountain trip.