I’m into an ultra-compact niche of the UL backpackers and now use a 24L daypack for comfortable ~3nighters backpacking. For ‘bike-packing’ I intend on using this
frame bag + dry bag (~27L total) set-up for gravel bike tours (eg, GAP/C&O), when I get around to it. I too prefer limiting the number of smaller bags for easy of packing and I also found I didn’t like the handling/stability of traditional bike-packing seat or handlebar bag set-ups. The dry bag backpack will max out ~10lbs of pricier UL camping gear/clothing, while the frame bag would store the heavier, inexpensive, easily replaced stuff (lock, water, food, spare tubes).
I figure the ability to quickly release and backpack the dry bag will also help negotiating tough single track, and prevent theft by taking the dry bag with me (eg, in restaurants or while sightseeing).