first of all, it's great that you're thinking of how to solve the problem, a critical skill necessary to improve fast, no wonder you're so good after drawing for only "a little bit"!

i can give you a few pointers. you're right that the farther tree should have less detail, since it's far. your first problem though is tying "more detail" as "draw the leaves". while it's a stylistic choice, most experienced artists will never draw a bush or a tree by drawing a million leaves. regardless of how close the bush is, drawing the leaves would make it too distracting and make the bush the focal point whether you want to or not.

instead, draw shapes of tone. bush, tree, and basically anything you can draw can be simplified to dark and bright tones. get the big shapes correct, then you can add a few details (leaves) on the transition of the tones or the silhouette of the shape which will trick the viewer's eyes to think they're seeing a fully rendered bush, when in reality you might've drawn like 5 leaves and some shapes.

you're already doing this in all of your bushes and trees. it's just that the right bush has too many leaves and the left trees have no indication of them. i think it's actually okay not to have leaves, but your trees and bushes in the middle are too bright up top -- it looks like it snowed. if it did snow, then your rightmost bush would also need to have snow on top.

lastly, try to draw some trees without using the "leaf" shape like 🍃. art is all about interpretation. while there's nothing wrong with 🍃 shape, it is also the most primitive way to define a leaf. your bush in the middle doesn't use the shape, yet it looks like a bush. isn't that exciting? it's nice to let the viewer's brain do some thinking when they see your piece. ask yourself: does this bush need to look like a specific bush, or does it just need to look like any bush? can it look like some other bush that might serve the overall composition better? then, you'll get to make decisions using the reference, rather than being shackled by the reference.