I’m one who can arch the back and hunch the shoulders at the same time. The arching of my back is better the more open I can get my hip angles. The hunched shoulders is more of my conformation and amount of muscle I have across the top of my back - I’ve always had close to “swimmers shoulders.”
My abs are not sore after riding or doing a lesson or clinic. I usually ride 5 days a week and workout 4 days a week (of which 2 days are very focused on my core). And to further comment on working on your “abs” or “core”…The core class that I do works you from your knees to your shoulders. A lot of people forget about all those muscles that are not the abs that make up the core. I found strengthening muscles from abs to knees really stabilizes my position. And now I can really HH from my whole core, but also break it down depending on what the HH needs to accomplish - for the passage, I think about my very low abs and pulling them up towards my belly button, but making sure my back is loose. Piaffe is more upper thigh + abs and a little bit of back. Pirouette canter is a whole core HH…
I can ride the hind legs up under if I’m careful about keeping my pelvis neutral. I used to have a bad habit of switching between a driving seat on my back pockets and then rotated too far forward - both of which inhibit bringing the hind legs under.
Although too short at 5’ 2" to be an “ideal” body type for a dressage rider, I have a shorter torso and long legs, especially in the femur. I’m also able to ride with a very open hip angle, which makes me look taller. And I have a short-flap saddle. I still only get a few inches of actual leg on my pet dinosaur below the saddle pad.