Figuring out what you want to carry and then the bags is a good idea.
I have recently embraced a mixed set up of traditional and bikepacking bags to suit my purpose and flex them between various rigs.
The frame bag can expand if wanted or zip smaller and allow two water bottles in the frame.
The panniers on the rear are designed for the front but I don't need the large volume of rear bags so they serve well.
A simple dry bag with a harness works as a handle bar bag and I sometimes use a gas tank bag for my camera and snacks.
Both bikes have rear racks and if I wanted to forgo the panniers (single track or very bumpy roads that would shake the panniers loose) I can bungee a drybag on top.
Even though I have a seat bag I dislike them for trips as they hold less volume than a saddlebag and risk flopping down on the rear wheel if the straps fail (seen that happen).
With panniers, saddlebag, drybag, frame bag and gas tank bag I can flex the system for equipment size and terrain needs.
Here are two pics using the same gear on two very different bikes
