Mustang just means feral horse. The conformation, ability, and type vary widely depending on what horses have joined that particular herd over the centuries. Some mustang populations have a lot of draft blood, others have TB blood from the old cavalry remount days, and there are some isolated populations like the Kiger in Oregon that appear to still retain Iberian characteristics.
Feral horses do seem to self select for efficient body types and to breed down to a practical 14 hands or so over time. In general big gaits are not an efficient use of energy. Obviously gowing up on the range makes for a canny and sure footed horse though I expect years in a holding pen would not be good.
I had a feral horse as a kid. She came in from a roundup on an Indian reserves in another province to a dude string in my suburb, where I bought her well broke but very unschooled. She was a completely fantastic all around kid’s horse, brilliant trail horse, could go go go all day ( I also fed her a gallon of sweet feed every night. It was a long time ago). She had good functional confirmation for a small horse, and was cute. Some of her herd mates though we’re butt ugly.
So I don’t scorn mustangs in general. But you can’t say with certainty anything about the potential of mustangs in general because they come from very different populations.
On the other hand I have been on my own dressage journey for the past decade with a big Paint who is athletic, uphill, smart, and has a great canter. And she can passage. Once a year she will passage for about 4 minutes when she is let loose on her vacation field and sees her old herd for the first time. There is no way any one will ever get that out of her under saddle. It took her a long time to get balanced in lateral movements in hand. She came to me with a sewing machine trot and after several years would do a decent working trot that tracks up, but the amount of work that took would have resulted in a more dressage talented horse doing a true extended trot.
Anyhow I say all this to make the point that while every horse will benefit from dressage training but the physical limits will become apparent sooner rather than later. I watch my coach’s Iberian cross horses and they are so much better adapted to dressage, they can naturally do lateral movements as soon as they figure out what you want, it’s just built in.
And gaits good or bad in a video where the horse is super excited do not tell you much.
However, OP tells us she has a herd already and a farm, and is just interested in experimenting so the stakes are much lower than if this was a one horse owner paying board.
I would just caution that taking on an unhandled feral horse is not what I’d consider a physically low risk activity for a rider in their 60s wanting to slow down a little (considerations I personally also face) so I hope OP has some good help IRL with the initial backing and breaking.
