I just took the bold decision of trying to redo the game using Q3D instead. I've been playing around with that plugin again, and finally starting to understand how it works. I'm still going to keep an isometric perspective, and I can reuse much of the assets since they are already quite low-poly.
The reason I'm going to switch over from pre rendered sprites to 3D is mainly because everything is so much harder to do in 2D isometric at this point, as construct 2 is mainly built for platform or top down perspective, i have to do a lot of heavy calculations, for arrows, and using up a lot of memory budget just for characters animations in 8 directions.
It's a little step back in the development, but It seems like i can keep most of the game mechanics, the multiplayer functionality and much else. I'm just going to switch all the graphics and use real-time 3D instead.
* I can easily use several color versions of the character, by just switching the diffuse-map. (instead of having multiple renders of each character)
* Arrows are going to behave more naturally, and not so heavy calculations to get corrections for isometric perspective.
* I can reuse a lot of assets in different angles and colors, instead of having to render 1 sprite for each object. in the orientation that I want it in.
* I don't have to worry about too many draw calls and Z sorting.
* Character can now rotate freely, and I'm not limited to 8 steps, which doesn't feel natural in a precision archery game.
Performance would be pretty much equal or better, depending on how detailed I do the map, but as I'm not using complex shaders with bump, normals, specularity etc etc, It's going to be pretty fast.
Visual Quality is going to be pretty much the same, except the characters which I have to use a bit more low poly version of, but It should be fine, since they are quite small.
Wish me luck... I'm keeping the old version of the project for rollback if things go south, but from my tests so far I think Q3D will be better.