Is it possible to "paint" heightmaps onto parts of a plane just like you can paint color?

I am trying to make a more detail version of a world map I am making for something, I have used the inbuilt A.N.T. Landscape maker to make some good and very detailed height maps (52mb png files!). My current rather pathetic attempts to make realistic looking islands is as follows:

It doesn’t have to be super high res with individual trees and valleys on, but as you can see, my own efforts from using the sculpting tools is rather bland.

In much the same way as it if possible to paint colour onto a terrain (plane), is it possible to manipulate it’s height by painting height onto a section of it?

I know you can apply a heightmap to a plane as a whole. But I would like to use several heightmaps on the same one and do it in real time if possible. The only solution I currently have is to make each islands’ height map in photoshop, then apply ot to the terrain and hope it doesn’t look awful. Though it certainly would be hard to look any worse than what I already have!

Just did a short test with the displacement modifier but the interface updated not in real time. Had to keep pressing tabulator for an update (2.81a). But why not just in sculpt mode. There is also a texture version of A.N.T.

Because in sculpt mode you only have a simplistic brush and raise, lower and smooth an area which is what gave me the result in the image. You just cant make landscape that looks any better than a childs play-dough drawing with it.

I should also add the reason I cant just use something outright made by A N T is because it only makes things in one style (mountains, lakes, canyons, or rivers). You cant use multiple styles on the same generated mesh.

Whether it looks like a child’s toy drawing or real is up to your skills or rather the effort you put into it I would say. There are tons of landscape alphas out there you can use as brush in skulpt.
I personally always try to start with generators and then improve the result by hand. As I said before, textures and the displacement modifier just don’t work in real time.