A bitless bridle can be harsh depending on how it is set up and also how the rider uses it, the nose has lots of nerve endings and is sensitive. Not all horses like bits and also not all horses like bitless bridles.
The severity of a bit depends on the hands it's in! A harsh rider can make a simple double link lozenge snaffle painful for the horse, a sensitive rider or a certain type of horse can make a running gag bit soft as butter!
But usually what's pretty mild are bit that are direct action and have smooth mouthpieces with no sharp bits, a double link snaffle with a lozenge is good (it doesn't have a nut cracker action like a single link snaffle does), a rubber Mullen mouth bit can be mild.
Or a leather bit you can find them on etsy.... They are handmade and work lovely it is a bridless bit but you can use a bridle its soft of your horses mouth and over time it shapes to said horses mouth and it most comfortable.