I owned horses for over 30 years, and I lived with my horses for many years.
Then after that I was fortunate enough to find a really decent riding stable with lessons. Students can lease a horse when they advance, the lesson horses are usually quite tolerant of their imperfect riders, and I have an excellent riding teacher.
In both cases, the horses turned out daily, with a pretty full social life, and well fed. They obviously enjoy their life as a horse.
I get the impression from these horses that they consider being ridden or being used for lessons as ENTERTAINMENT as much as it is work. That while they may have extensive turn-out with other horses they get a little bored, and that the humans are interesting enough so that the horses are not as bored around the humans. Then they get turned back out and go back to chilling out with their friends.
And good grooming is much appreciated, especially by the older horses.
I do not consider lesson horses carrying children, with breaks in between rides, as overworked when used for lessons 6 hours a day, so long as they are fed appropriately and their hoof and veterinary care is regular and done to a good standard. Horses are perfectly capable of carrying children that long in lessons, where the teacher makes sure that the students do not overwork the horses (excessive galloping.)
But then I spent my grade school years in countries where there were plenty of working horses in the streets, so I got used to seeing horses working. Light cart horses pulling two wheel to four wheel wagons for moving and selling vegetables and moving various commodities around, on pavement in the capitol city of the country. In Uruguay the working horses turn out was being let loose on the suburban streets to eat the grass on the roadways (the yards were walled) and to rummage through the garbage cans.
Our horses in decent stables have a very easy life. Of course a lot of them are obese and they really need to work out more.