Just want to point out that horses can carry a bit with their tongues. A double-joint snaffle will tend to apply more pressure to the tongue and less to the bars. It is obviously possible to injure the horse's tongue with a thin snaffle used harshly, but a well designed bit used lightly mostly just rests there.
Another option to consider is going western, where one typically uses a curb bit - but rarely. The bit below was my mare's favorite - based on what she acted most calm, most relaxed, but also most interested in continuing forward with a cheerful attitude:
Used like this, it is not in any way a harsh bit:
The bit that seems to give best results with my current horse is this one:
If he is feeling nervous, or if we are going at some speed, he prefers constant contact, at least for a few minutes. Like most horses I've met, mine tend to pick up the bit. When you place it in their mouth, they wiggle their tongue and you can see the bit go 1/4-1/2 inch higher, where they then carry it during the ride.
The horse I currently ride was previously ridden extensively in a bosal. He also has a lot of white hair on his face, near where a bosal sits. It is the only place where he has white hairs. I take that to mean a bosal can be harsh, if ridden improperly.
I've also ridden him in several different sidepulls. He does fine unless he gets excited. Once excited, he seems to forget about his rider and tries to make the decisions by himself - which would be fine, if his judgment was at least as good as mine. But it often isn't, and then he is more likely to hurt himself (and me).
A western curb bit, ridden correctly, is arguably gentler than a hackamore. I won't go into why here, but they can give a very gentle way of communicating with your horse. They obviously are not meant to be ridden with constant contact for any length of time, which is why I mostly use snaffles with my current horse.
You cannot separate "gentle" and "pain-free" from the individual horse, rider and goal. Horses make choices, and horses don't watch YouTube videos on what makes them happy. Keep an open mind, experiment, and let your horse (when you have one) tell you what to use. The horse's answer can run from a simple sidepull to a curb. Some (Bandit) favor contact, while others (Trooper) do not.
If you want a few GOOD videos on bits and how they work, and have a couple of hours to learn, I recommend these: