Ah, very nice!
I would love to do a project like this. You may have inspired me

I didn't see the original, so don't know how much it might have changed already. From your last reply, I do like your 3rd attempt on the fir, and think it is the best so far. I love the general shape, where you have the kind of pointy way of depicting the branches. I think that's really effective.
As a "tree person" (Bachelor's in Forestry) 2 things strike me right away. One is that the trees all look very similar in color and composition, even though by their labels, I know they are very different trees. The other is that they all look about the same height, which again, I know they probably aren't. I would suggest much more variety in shades of green....unless that's part of "your style" to have a blast of green. Like I might make the fir (Douglas Fir?) lean a little more towards blue-green. Not a lot, but just enough to depict firs' true color, which is not really a bright kind of green (except on fresh spring growth at the tips maybe) (unless they're White Fir...omg, college was 30 years ago

...maybe I'm thinking of Frasier Fir). I think I might try deepening the color as well, but it's hard to say without actually doing it. It may be that just changing it a little bit more towards blue-green will do the trick? You could also vary the shades of green among the fir, for a really natural depiction, although I realize that's not how landscape architects usually do it.
Still on the fir, and using that group of 3 towards the lower right as an example... In reality, you probably don't have the entire crown of one that lies above the top tip of the next one (and that one's entire crown above the 3rd). I would suggest unioning the lower level/layers of foliage of all 3 trees, and maybe even the 2nd level/layer. And then keep the unique upper level/layers, which may well be touching each other. I think that might give the appearance of the fir growing close together with braches entwined, rather than one on top of another. It looks actually like a grove of fir. And I can't see it clearly, but it looks like there's a fir in the middle there too. Is that grayish color just dirt, where the grass won't grow because there's so much shade?
And finally, I think you could use the cast shadows to further emphasize the height of the fir, and the differences in height. Even though it's mid-day, and the shadows are very short, they would still be different lengths. ....unless they are all about the same height?
One of the few trees with which I am not very familiar, is the Filbert. So I don't have an instant mental image of the leaf color, or general habit. But I see from the photo on the webpage you linked to, that it looks like fairly a short, wide shape, with not super dense foliage. I wonder if there might be a way to....I don't know, depict the difference between the density of the fir foliage, and lighter less dense filbert? Like maybe add some Noise to the filberts or something? Or at least change the color to a little more towards grayish green (at least that's the impression I get from the photo).
And also the same tip about them being side by side instead of one on top of the next. I wonder if you could union the foliage, and have the branches and radial gradient still individual? It's hard to tell from the photo, but do they really come to a point at the top like the firs, or are they more flat topped (like a dogwood)? My impression from the photo is that they are more flat topped, but I'm not sure how to go about depicting that. I do think it would help to give them a more flat topped appearance, I'm just not sure how to do it....
I like how you've depicted the branches of the deciduous trees, by the way. It's also very effective. I can't quite tell if the branches have a gradient? I'm not sure if this would work, but you know how the branching habit of the filbert is different from that of the cherry? The cherry has the strong central trunk with smaller branches, and tapering towards to the tips, just like in your image. But the filberts don't have a strong central trunk, and in fact one of them looks to have multiple stems (trunks) and there's not a lot of taper. I was just wondering if there might be a way give such an appearance. Maybe the shape of the branches....like if they didn't go out to a point, but were more rounded? Or maybe if they didn't meet in the center? Or using a reverse gradient (more transparent towards the middle -- opposite of the cherry).
Ok, and the smaller, darker ones with red spots and one with yellow -- are those flowering shrubs? I can't read their labels because they are so small. Depending what kind of shrub they are, perhaps a different shade of green might help? Or not

I CAN read a couple of Bay Laurels though. I'm not very familar with those either, but for some reason, I'm thinking they have very dark green leaves....but not sure at all.
And I think a lighter green, and maybe more towards yellowish green for the grass, would help to make the trees pop, and hopefully help give them more height.
And one last comment. Is there going to be a legend for the plants? I'm wondering if you used several different shades of green, if you could use a legend, and not have the text labels. Not that I think the text labels detract from the image, not at all. I'm just thinking more like a map, I guess. But anyway, just a thought.
Ok, gotta rush away. But I may have more thoughts later. What a fun project though! And congrats on the nice work so far

All best.