I grew up carrying musette bags (from Turin Bicycle Co-op) and graduated to messenger bags of all shapes and size, so proabably not the best for advice. The few times I use one, it sill feels like I a sticky place on my mid back where the bag sits after all those youthful years of practice.
As you have already imagined or experienced, if the bag swings around and hangs down while riding aggressively on the drops, it gets in the way.
I'd say that the modern messenger bag is pretty well designed, with single shoulder and retention waist strap to keep the movement at a minimum.
That said, a better design that carries the mass more symmetrically (something you're back will appreciate in your doteage) would be the modified lumbar "fanny packs" that use 2 shoulder straps and and a waist strap.
In the end, as soon as I "graduated" from rackless racing bikes, I stopped carrying any weight on my body. Stuff I need to carry goes on the rack, or if heavy, down low in small panniers.
Crossed paths several times on the GDMBR with a well-loaded, young adventurer from Vermont who carried a chunk of equipment in a lumbar pack. By the 3rd time I met him, he was trying to figure out how to get that weight off his body and on the bike..., anywhere on the bike. If nothing else, think about that extra mass pushing your rear onto the saddle with every bump.
Ride on.
