Last month I bit the bullet and bought a Kriega Trail 9 ("T9") backpack. https://kriega.com/riderpacks/trail9
As I am sure that you know, Kriega kit is both stupidly expensive and absolutely superb. The T9 ticks both of these boxes.
Really designed for trail riders, it has a compartment designed to hold a "water bottle" (5 litre compartment designed to hold up to a 44.5 litre "water bottle") and a separatre 4 lite compartment, intended for "bits and bobs".
Then there is an extra "neoprene" hexagonal web expansion pocket that you can just stuff things into, giving a little more storage.
In practice (for me) it is simply a really good little backpack, perfect for carrying my phone / glasses / cheque book / spare contact lenses etc etc, and - if necessary, sticking in a bottle of water (not the sort of "camelback" system that it was designed for - just a simple bottle of water.)
Now, it is stupidly expensive, but - my word - it fits so well (a four or six point harness system does not give much scope for it to move at any speed), is as waterproof as you would expect for Kriega and, best of all, you can actually connect the Kriega US 10 expansion pack to it: https://kriega.com/sports/touring/us-20-drypack19a
to give a 19 litre carrying capacity.