Not that you don't already know this, but I have to start by saying I adore your art and these are still so stunning to me! <3
In terms of being stronger for Res Style, I'd have to agree with all the things you already pointed out. Lineart and shading should be v crisp and cell shaded. This means no flow or opacity should be used.
I think your posing for these are quite nice!! The silhouettes are strong and you are using up the square canvas space well without filling it up too much and making the art itself look square & confined.
With your ex of the feelers missing on the tail: I think that's actually perfectly fine to do in order to make the piece feel stronger and less cluttered. On the flipside, feelers placed in those missing spots could still work out if you are careful not to break the silhouette and also use depth to control the eye's focus on the face.
Speaking of depth, I think that is probably the one area you could focus on to make your work even stronger. Not that it's bad at all, but it could be exaggerated more to help these smol graphics.
For example, I'd suggest that the Uldavian Ahea's tail goes more into shadow as it goes back behind the head all the way up to just before where the tail starts to come to the front again. A tip to help with this as well is to be selective with using any highlights in those areas.
We want to use highlights to focus on or bring an element forward. So putting highlighting with those shadows that are things pushing back can fight with it. For those push-back areas, you can use the shadow's edges to imply some kind of highlight without using the actual white/brightness of the highlights you're using (I hope that made sense!).
For the Gold&Silver Ahea, I'd suggest putting more overlapping shadows for the body just to, again, exaggerate how it's going back in space/perspective just to really accentuate that the head is turning and coming back forward at us. Same with the tail, even though it's in front of the body/head, you could exaggerate the shading midway to push that the end of the tail is coming forward even more.
Another great tip for helping with depth is to use cast shadows! This can really give us a stronger sense of where an element is in space.
Proportion-wise your work is really strong and very close Res Style. If I were to nit-pick it'd be to suggest being careful with major body parts like Ahea's feet, might do better for them to be ever so slightly more thick. When it comes to OAKs though, there is a little leeway considering this could simply be an Ahea with thinner feet. :3
I did some quick shading adjustments just to give you examples of what I'm talking about:
(Hi hello I streeeeetched the page!! Huzzah ovo)
Last, for your item: Yes we absolutely use a blacked-out silhouette/lines for those thinner areas if it creates a better look than an ultra thin filled-out area. Depends on the look you're going for. Additionally, I might use a dark color instead of black just to help imply the color it should be. So for example, a super dark green for that flower stem.
Wow that turned out a lot longer than I intended, sorry!! I hope this helps! Feel free to use this thread to continue talking about this. :3c