Nice looking project I always love seeing others doing more painterly maps - they're my favorite style, but more of mine are panoramic than top down or isometric. Another memorable painterly cartographer here goes by Viking, you can check out their work for some inspiration... this is one of my favorites: https://www.cartographersguild.com/s...619#post224619

For improving a painterly style, I like to study actual paintings (of landscapes mostly but that's not the only thing that can help) and do studies of terrain based on aerial photos, videos and maps (like Google Earth satellite and 3d views). I would also recommend doing all of that as well for painting forests in particular...as to a specific forest style I don't have a recommendation; I'd just try stuff until I found something I liked for it.

Have fun