There's a lot to think about here and a lot will depend on your bike as well, particularly the size of it. With smaller framed bikes there's a lot less room for bags and seatpost packs in particular can be very problematic. In general you'll need over 15cm of seatpost visible to accommodate the majority of seat packs available. I only have 12 cm's which has caused a few issues in selecting a suitable bag. I currently use a Topeak bag on my Gravel bike, but the problem is the outer bag is not waterproof and the inner drybag that they provide isn't shaped to fit the entirety of the bag leaving gaps which makes it a pig to pack properly. I have a Restrap bag on the MTB and that is much better, it's a holster style bag, whereby there's an outer harness and a single drybag that slots into it. This is a great bag, really well built, easy to use and is rock steady.
With regards frame bags, you really need to think about water and water storage. It can get very hot in the Aps in Summer and high twenties/low thirties are not unheard of. I tend to go for over 2 litres of water and with the gravel bike I like to carry that in the main triangle. I have room then for a half frame bag, which currently is the Excellent Apidura one. I originally owned a cheaper Topeak one, but the attachment system was flawed and a lot of weight was placed on the zip, which ended up breaking. This Apidura one has multiple attachment points and is generally much better constructed. I also have the Apidura water bladder which sits inside the frame bag and a drinking tube can be attached which works really well.
On the Mountain bike, I use an Ortlieb bag which is completely waterproof and I deliberately have the roll top one so I have no zips to worry about breaking.
If you don't want to use the main triangle or frame bags for water, then an alternative is to use feed bags which mount either side of the stem. I use this method on the mountain bike because it keeps the bottles up out of the way of mud and poo. With feedbags there really is only one manufacturer to consider and that's Revelate Designs feedbags, no other design comes close, these bags are fantastic. They have the ingenious one hand opening closing system and lots of other genius design features.
Front roll can also be problematic with a drop bar bike, you really don't get much space between the drops on modern bikes. I actually have 500mm wide bars so can squeeze a 8 litre dry bag in there in a Topeak front loader harness. Narrower bars though lose some of those litres.
I also have a Revelate Designs Mag Tank on the top tube, again a brilliant bit of kit with a fantastic opening system, no zips to break or worry about. The only downside is that this is not completely waterproof, but as long as you are aware of this, then that's fine.
The only other bags I use are the fork bags on my gravel bike. I use the Apidura fork bags attached to Blackburn cargo cages. Brilliant little bags that are fully waterproof, easy to attach and detach and don't budge an inch on the roughest of trails.
If you look for my posts in the Travelogue section, you'll see the numerous trips I've ridden in Alpine regions. I don't really worry about any special kit, I actually carried more cold weather/wet weather gear this summer in Scotland. In the Alps I just carry mostly hot weather kit, shorts etc and a lightweight rain jacket. The biggest issue can be water, depending on where you are going. Some parts of the Alps are Limestone/Karst landscapes and natural water sources can be scarce, meaning you need to carry more water. I've also started to use High Five hydration tablets, which have had a real positive impact on my riding and recovery.
Here's my Gravel bike set up for the Swiss Alps with full camping gear, one man tent, lightweight sleeping bag, clothes, cooking kit food etc..
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Here's the Mountain Bike in Scotland this year with a slightly fuller set up than I would use in the Aps.
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