What's his diet?
My horse with bad ground manners is oat/corn/soy intolerant. Once I removed all those things from his diet, I quit taking him to the Woodshed everytime I blinked.
He will still test me but it's infrequent and when I say "Rusty put your mask on", (yes his name really is Rusty) he now comes up to me, head down and does his darndest to dress himself.
I never realized what a difference diet makes in the most subtle of bad behavior patterns until I discovered this horse's food intolerances.
He also has seasonal environmental allergies; he's on herbs for that from April thru September. Getting those under control also helped his behavior issues