Well, your horse is naked, I don't know about you.
I loved your post. You are so forthright about what you know and want to know. Refreshing to hear such an open mind.
I love Sylvia Loch!! Love her, love her. She is one of the few people who talks about the seat being more than the two seatbones. She harkens back to the old masters and I just love that. (have you ever seen a video of Nuno Oliviera riding? Goggle that. There aren't many in exhistance as he died a good while back but his seat is the most beautiful I have ever seen. )
Have you looked into a treeless saddle for you horse? Since she is so round that a treed saddle will slide anytway, would a treeless work? The Ansur saddle is a good one for work in dressage as it puts you in a very upright, aligned position. If you are really balanced on your horse, then the saddle will be unlikely to slide anyway (tho no guarantee).
As for warming up; It is harder for your horse to round up under you in a sitting trot if she is not well warmed up. Since warnming her up the traditional way , lots of forward trotting in a light or posting seat and her stretching down , is not so easy for you, you may want to warm up at the walk only. You can do a forward , swinging walk with her stretching and reaching down/out for a good 10 to 15 minutes before you go into trot.
Be sure that you don't just go mindlessly around the arena, stuck to the wall. YOu would want to do circles and figure 8, back up, stop-walk-stop transitions, shoulder-in and "freewalk". All can be done at the walk.
The only thing I can see as a bit of a hold back is that if you aren't comfortable with your horse really trotting out when you are on bareback, then that could be a little limiting , as you want to really encourage a "forward" mentality in a dressage horse.
I admire your seat, I used to ride bareback some, but am just too fearful of coming off to do it much now. (I am 52, so I guess that isn't so unusual)