I use Clip Studio Paint. I've made some brushes for mapping. They aren't really "released" and the closest I have to a "tutorial" is the videos I screen recorded. But you can have my brushes. I don't know how long I'll leave them up. "soothing watercolor 4" is a blender, not a paint brush, you'll see me use it a ton, and I THINK these are the trees I usually use (though I make assorted variations on the sets depending which trees I want on the map, basically you can just use the concept of the brush settings and add whatever trees you want in there). https://www.dropbox.com/sh/fowi3xlv9...DPcsc0v5a?dl=0
anyway, these should all be brushes I made myself, so my usual terms apply, which is "credit is nice but not required, use them as you want, commercial use of your created map is acceptable, don't sell the individual resources / something that is entirely dependant on the resources as the reason it sells"... for example, don't literally just stamp them and sell the stamps as a "token kit". But assuming you have used them as intended, as one of the many tools included, you can make a map for a paid game, sell the map, put the map in a book for sale, put the map in a monetized YouTube show, etc.
I'm a little whingy about sharing my exact trees, so I pulled them off my old computer rather than using the most current set of them, because I'm constantly adding new little trees to the stream, but eh, I mean, you get the point, you can just add in as many trees as you want and the settings jiggle them around a little so they don't generally appear repetitive. You can definitely reverse engineer these brushes and make a wide variety of other brushes. These are more for overworld mapping though.
The best battle map resources will be photo-based and you'll find a large collection by Bogie on this site, but there are many others by other artists. Mark Oliver has collected many of them over the years in a compiled downloadable state. They aren't specifically for any program (though Mark made them installable for several specific programs), they're just stock art. You can use stock photos in Clip Studio Paint. The difference between that and my brushes is, I believe most if not all of these brushes are set up so you can change the color of the outline and fill by changing your foreground and background colors.