I am bringing this thread back to comment on the stirrup length issue and to make a couple of comments on training in general, since you ride with a dressage instructor.
Stirrup length is not a cut and dry issue, not in the European style of instruction at least. Rider’s stirrup length should be adjusted primarily to his/her conformation, their horse’s conformation, the work, and the terrain, the work is to be performed. The saddle should be able to accommodate adjustments (to a point, plus/minus a couple of holes). If it can’t, it might not be the right saddle.
I am a spider with very long legs, yet ride different lengths on different horses to accommodate their conformation (in dressage saddle). I have to ride “shorter” on my round horse btw. Furthermore, I have to adjust to work- flat work- long, cavaletti-shorter, country ride- shorter also.
I had to respect all above even when I owned and rode in western saddle.
If you really want longer (for your situation), stronger, and more stable leg, get seat lessons or at least ride frequently without stirrups under supervision of your instructor (10 minutes within each lesson perhaps).
I encourage you to trust your instructor as far as your stirrup length goes, because she/he can evaluate you better than any of us on COTH. The same goes for the saddle fit (for the rider).
I see, you are learning from all sorts of different sources like most of us.
Since you take weekly lessons with a dressage instructor, I think, you might consider having a talk with your instructor and build some plan together. Do you trust the instruction and follow it? (because they can tell) Do you get homework (you should)?
What school of thought does he/she belongs? Who are the role models for your instructor? Not all dressage people ride with pounds of pressure in their hands, use shortcuts, produce “braced” horses, etc. Flexions, “twirlings” are well known concepts also.
It is OK to put different ingredients into a pot, but we all need a solid base. Progression pyramid has not been put together by dressage theorists for nothing.
Sometimes, too much experimenting hinders the progress, because your trainer might have some development plan and then you play around with different concepts, and horse is getting caught in the middle of it. It can become frustrating for everyone- horse, rider, and instructor.
If I were you, I would stick to a person I could trust, commit to their instruction and riding plan for some time and see what would come out of it.
I would discuss with them any plans for little detours ahead (like getting some idea from a book or from video). Many instructors appreciate a dialogue, but you have to have some common understanding of where you are going.
Whatever you do, enjoy your horse! He is lovely.