Glad to see you are working on long tiltless strings with 3's thrown in, rather than some video with just a bunch single tricks and a few short strings. You are definitely approaching the sport correctly from that angle.
In regards to your posture, I used to play a lot like this and I've been working on more relaxed form.
Here are my thoughts.
What should help you with the hunching is to just slow down and relax. It seems like you are tense the entire run and exerting a lot just to do basic dexes. This is also causing you to slam the ground on each contact rather than be light on your feet.
To fix this, work on your basic 2>2>RPT drills (e.g. clipper > pickup > RPT) and trying to do them as slowwwwwwwwwwly as possible, delaying the bag as deeeeply as possible. Don't worry about number of contacts for this, just work on:
1) Being RELAXED. Relaxed muscles. Relaxed mind. No tension.
2) DEEP delays. Let the bag die every contact.
3) Landing on the balls of your feet on each contact.
4) RHYTHM of both your tricks and your breathing.
What helps me a lot in employing these tactics is to do runs in my mind after warm up and every so often during the session if I am starting to tense up and rush things again.

Using all 3 senses, run through a visualization of yourself playing, from the 1st person perspective (not 3rd person). SEE each trick happen slowly and controlled. SEE and FEEL the bag delay to a stop on each trick. FEEL your breathing rhythm during the run. FEEL relaxed and loose, enjoying the entire run. FEEL and HEAR the balls of your feet lightly landing each tricks (no thumps of your heels

) and so on. The visualization should be in the first person and use all 3 primary channels (seeing, hearing and feeling). A lot of times the things we have trouble doing are the things we can't clearly create in our mind's eye.