I know it’s terribly unfashionable, but at least consider a rucsack. If your kit is light enough it won’t be a burden, and you get full technical ability of your bike too.
+1
I hate backpacks with a passion on the bike, but as my local loop is ~27miles I’d struggle to do that on 750ml May-September so take the Camelback (but it’s minimaly packed, pump, tube, tool, rear light and sweeties). With a bit of planning I’m sur eI could stuff it with the lighter Bivi kit. Infact it’s probably the best place for a lot of the small stuff in the absence of a frame bag as bar bags and saddle packs work much better with big bulky items that hold their shape when compressed (sleeping bags, mats, etc).
I think you’re going to have to sacrifice something, either the dropper post, the FS bike or both. I’d lose the dropper when bivi’ng, you’re not going to be getting overly ganrr with a bar bag 20+ miles form anywhere!