I've asked on several threads how one mounts a bike packed like Alan's pictured above with a high, protruding butt bag. Answers ranged from "lay your bike on the ground, step over and lift it up" to "bend your knee sharply and swing leg over top tube in front of saddle". I board my bike using the traditional "cowboy mount"...left foot on pedal, swing the right leg over from behind. At age 70 I lack the flexibility for any of the above methods with a bikepacking seat bag. Even in my youth I don't think I could have used a butt bag, thus I tour with panniers.
If I were to embark on a light tour as you have planned on a road bike without racks I'd use a more traditional saddle bag that favors width over length. Maybe something like
this? (It may require a saddle with bag loops) That way I could still mount the bike normally. Maybe also use a small handlebar bag. A backpack you may regret.
Here's how I light toured in the '60's using hostels. Two small panniers with a HB bag. Yes, that bike (a road bike) had dropout eyelets for a rack but there are ways around that now. It clamped onto the seat stays at the top. I prefer the weight low as opposed to a saddle bag...
'68 WB Hurlow ^