I hunt elk in WY in the same manner, guided and on horseback and have used various daypacks in the 1200-1500ci range. We hunt end of Sept to early October. Typically will put raingear in the saddlebags to save pack space. In the pack I'll have: 1.5L water bladder & 16oz nalgene, steri-pen, first aid pull out, fire starting pull out, spare rifle & pistol ammo, GPS, shooting sticks, headlamp (2), possibles pullout(TP/spare batteries/repair tape, knife sharpener etc), lunch & snacks. There's usually room to stuff gloves, neck gaiter, beanie, etc. if I shed those things throughout the day. I don't carry spare clothing as we are back in camp every night.
Any bulky outer layers that get taken off during the day I will typically roll up and tie up behind the saddle . I also laced shock cord on my new pack (MR Front) to hold any outer layers that I want to quickly stow, as an option.
I too am trying a new clothing system (First Lite) this season. Been a Filson wearer for the past 20 years so looking forward to trying out this lighter weight wool!
One thing to keep in mind if hunting on horseback is how are you going to carry the pack while on the horse? I do not wear mine while riding, but hook it over the saddle horn on the off-side. Because of this I look for a pack with a decent sized grab handle on the top that will fit over a saddle horn and one that is somewhat narrow in profile. Personally I don't want/need a hip belt or waist strap and it just adds bulk if not really used. When we stop I'll pull the pack off and put it on. I've seen a few guys wearing their packs (typically way over sized/packed) and it just adds a whole other dimension when mounting and dismounting, not to mention riding through any thick timber. Plus you have to support all that weight when riding. IMO hunting via horseback out of a base camp every day, a huge pack just isn't needed.
I don't know how you're traveling but if you're worried about the Flash 20, you might think about throwing in a smaller day pack. I use mine as a carry-on when flying up.
Have you discussed it with your outfitter?