For years I carried a powermonkey extreme 9000mAh battery bank, it was good enough to keep my then smartphone going all week, and I topped it up with a solar panel.
These days my smart phone has a larger battery, which it gets through faster, so I have to carry more by way of battery packs.
I have a dynamo with an usb-werk, which I can use for topping up the battery packs, but it suffers a bit from the fact I'm still walking up some hills...
The battery packs I have are a 20000mAh pack I was given, this has a fast charge output, plus 2 normal high power outputs. I then have an Anker 5000mAh pack. The later I tend to reserve for when I need to give my wahoo a boost and don't want to faff around with the big battery. or as an emergency charge for my phone.
I have 6 devices that I need to charge:
- Phone
- Camera
- Irridium device
- Wahoo
- Battery bank 1
- Battery bank 2.
I carry 2 chargers with me a 3 port Rav power charger, and a 3 port ikea charger. At least one of which is easy to access at the top of a bag, so that I can charge 3 devices for an hour over lunch if I need to, and find a cafe with a suitable power socket.
There is a strong argument as you say for having multiple smaller packs, as you can typically charge them quicker. But I notice that Anker do a 25000mAh pack that has 2 inputs, so you can plug in 2 usb cables, and charge it faster. I've not tried it, but it looks like a nice option.
Generally speaking, you're going to find life a lot easier if you have more power than you need, rather than less power. This is a situation where the downside of over specing isn't really that big.
Good luck, and let us know what you go for.
J