Heyhey @ChrisConnor - thanks for your patience 
There's been a lot of feedback given since I last checked, so perhaps other artists have answered a few of the questions you were asking me? But in response:
-> For sure, experiment with what works best for your vision
It sounds like you're doing that with Theo's clothes. Have you decided on the style you'd like? I checked out the styles of the artists you listed, it seems like a flowy, gradient style would best suit your goals (as opposed to the two/three toned, bold outlines).
I do this style by using a lasso tool to outline the subject (shirt, pants, whatever), and use the dry textured gauche, or a textured watercolor brush within the lassoed area. That way it has a nice clean outline but still looks vibrant and...storybookish. Same way you do the trees and bushes, but with a different texture. Perhaps that'll help you?
-> So, for your tulip fields - keep in mind that the sharper you make their edges, the more defined they look, the less the viewers' eyes are directed to the focal points - they're too busy bouncing around all the little details. If it's your intention to add details, great, you're on it, just make sure you have a plan to direct the eyes back to your intended focal points - in leui of details (it seems your characters are cartoonish and not intended to be detailed) you can do this with a contrast in values, hues, and textures. I know that with the artists you listed, all of their style has an attention to detail with lively environments/backgrounds, so I know it's probably a goal of yours to maintain that level of detail. Just something to think about.
Does the tulip field look flat? Not all the time, not in all places. Keep in mind that the further away something is, the "cooler" it looks, lots of blue hues. The closer something is, the warmer it looks, lots of red hues. That's just the way light works over a distance. The further away something is, the less the contrast in tones and fewer details are perceived, and of course, the closer it is.... the more contrast and details can be perceived
(sorry to repeat something you may already know).
If you want your environments to look like they go on forever (that seems to be the goal?), limit your palette, contrast, and details the further back you get, and use cooler colors (or fewer hues). For example, piece 1 - the hills by the barn could probably have the highlights a little lower to limit the contrast. The tulips could probably have fewer spots of dark green and incorporate some bluer greens the further away they are. Does that make sense? Same notes for Piece 3.
Experiment with those notes a bit and you'll find a way to give your pieces more depth.
I hear ya on being self taught - for the most part, I am as well. It's incredible what time, patience, dedicated practice, a few youtube videos, and constructive feedback will get you. Kudos to you for going the extra mile to invest in your craft by joining a learning community like SVS. You'll go far with this kind of dedication! You can do it!
(PS - I saw there was a redraw of Piece 1 - I wanted to comment on it but now it's not here, am I missing it or was it removed? Lemme know, I'm happy to give feedback on the changes!)
All the best! Happy drawing!
Shani