It is very clear how much you love your horse (rightfully so, he’s a handsome chunk!) so am confident saying you will not mess him up. Based off the videos you’ve shared, you’re doing a bang up job and you look to have a very good feel of him.
I don’t have a one size fits all approach to how I produce my horses but I will rattle off a few things I do that seems to benefit them all one way or another.
*I prefer to start everything in a cross country field - the spooky ones I’ll try to find a lead if I can to give them a little extra confidence. I feel like solid jumps etc are easier for them to digest when they’re green and it’s easier for them to find their rhythm
*I don’t drill small cross rails. Once they understand there is something there to go over, I bump them up to big X’s - it promotes a better shape and keeps them a little straighter
*I love a ground pole two or three strides after the fence. It teaches them how to rate themselves naturally without being overwhelming
*i really like jumping towards a the wall/fence and using that to encourage downward transitions
*if i do grid work, i usually do more ground poles than fences. I try to keep it to a minimum of two fences, never a vertical
*i prefer to encourage them to get the correct lead on landing rather than schooling a change when they’re green. If you have a wide enough arena to get straight before the fence (at a trot), I love setting a big X up parallel to the long side being mindful to weight my new outside heel on landing. If they get the right lead, I let them carry on for a 20m circle or until they’ve really settled into their stride. If they do not, no big deal, I do a slow transition to a halt on the long side letting them have a few steps at the trot, a few at the walk, and really letting my seat do the work. <-- hopefully that makes sense?
*I knot my reins so I’m not inclined to pull on their face and make it miserable for them
The long of the short: I try to do exercises that naturally allow them to pick the right answer.
FWIW - I like a flexible mullen with a D for the greenies. It’s soft, I’ve found most Thoroughbreds like a mullen, and the D helps with steering