I have a 16-1/2 yr old TWH that has been with me since he was 2-1/2. You could be talking about my Fella.
He quit doing all that stuff once I accidentally discovered he is oat/corn/soy intolerant.
I say accidentally because I my senior TWH was diagnosed with Equine Metabolic Syndrome 4+ years ago. I took all my horses off grain and vit/mins with soy in them to make my feeding life simple and it wouldn't hurt anyone to eat like the Type II diabetic guy.
Imagine my surprise when the guy with Snarky disposition mellowed so much that even my non-horse husband noticed the difference. Also, when I removed oats, corn & soy, it became the first time ever that I didn't need the riding crop in the bath bucket
This horse is also prone do dust/mold/pollen allergies. All Tri-Hist did was amp him up and make him a nut case. I now keep him on an herbal allergy blend during the warm months and he's fine.
He is so food sensitive that even adding TWO soaked alfalfa cubes to his supplements twice daily can make him a little testy.
He is also Generator-bred which, the older line of Generator TWH's were known to have a bit of a snotty disposition because they were bred for the show ring and way over-produced. My guy even inherited a less-than-Grade 1 club hoof from The Old Man, as he is referred to in my area
While Rusty is still not 100% in the personality department, he's improved by 90% to where I am ok with adults that are horse savvy walking around him.
However, he will always naturally be a horse who will take the Proverbial mile if you give him an inch. An intermediate rider wearing a "no-fear t-shirt could easily ride him, but a 20 year "experienced" rider that is really timid wouldn't stand a snowball's chance after the first 60 seconds.
I have no regrets hanging onto him all these years to keep him alive because even his mother would have sent him packing when he was at his food allergy worst. He's a fantastic trail horse and literally saved my husband from disaster on the trail once.
I said all that because I'm not saying your guy won't always need some "reminders" because that's who he is, but it may help to change his diet.
As soon as you said your horse doesn't like to be brushed, I thought he's probably got food allergies just like Rusty did.
I always used to say Rusty was uncomfortable in his own skin but I wasn't smart enough to figure it out on my own --- the other horse becoming EMS saved Rusty. Even his facial expression is a lot softer since I took him off grain.