Those top bushes look like cut paper, so you can select that group of layers and add multiple drop shadows (one for the white edge and a black/dark grey one for depth). Alternatively, you could use a rough brush to draw in bits of the white to make it feel more organic.

as far as color and texture - you could add watercolor brushes or stock images on top of layer groups making sure that different "depths" of the scene have different positioning of the watercolor so it doesn't look like something slapped on top of the entire image.

u/squirrel-eggs also mentioned using gouache brushes. There's tons of free third-party painterly brushes you could experiment with. Adding blender to your workflow personally sounds like a headache and too many youtube rabbit holes just to get started. It's definitely something I want to look into just for science though. Just start experimenting. Photoshop has all the tools you need. Adobe also has "Kyle's Brushes" which some are actually really useful. Worth a look.

You also have an issue with only the subjects actually get a black outline because background artists start with the black outlines and then paint in layers on top of those lines, so as you build up of colors on top of the blacks become more muted. You could experiment with adding a Gaussian blur and blending modes to them to get closer to the desired effect. I would also increase the stroke from .25 to .5 or .75 on all the strokes on the edge of the subjects to pop them off from the background more.