There are a few people complaining about the readability in AoMR but I think that is an issue that can’t be “solved”.
There are 2 big reasons:
1. You are used to AoM for over 20 years
If you have played one game for over 20 years you get used to how all the units look like on all the terrains and so on.
The same argument applies to AoE2DE, AoE3DE or any new game (AoE4). A game you have been playing since childhood has trained your mind for way longer.
Especially AoE2 players seem to be extra allergic to change. Things like regional skins have a lot of enemies because people trained their brain on seeing the same 5 units for every civilisation.
2. More details means more things for your brain to compute
AoM has very low polygon models and low res textures.
There are not many details that your brain has to filter.
AoMR has a lot more details in the units and the terrain. Unless you add some artificial highlights (like unit outlines) you will never be able to get the same readability as the old game.
You can choose to make a game that looks year or a game that is optimised for readability, not both at the same time.
If you look at other games in other genres. Early 3D games did not have to highlight intractable objects because the environments had little to no decoration.
If you placed a destructible crate in a game in 1999 you did not have to highlight it in any way since it was going to be very easy to pick out in an environment with almost no decorations.
But it you want to do the same today you need to make it very obvious that this object is different then other by making the crate look very breakable since there are many small none interactable object in every environment.
Game developers have started adding things like white paint onto ledges that you can interact with to communicate with players what parts of worlds are interactable. In an old game you did not have to do that because the number of objects a player could see at once was so small that they could easily tell which ones you can interact with, or at last try all of them relatively quickly.
Conclusion
Or course there is always things that can be improved to make the game more readable but there is a limit.
If you want realism you can not have perfect readability.
In real life a lot of military strategies evolve around distracting enemies or hiding from them.
Uniforms had banners, strings, flags or other details added to them that would catch the enemies attention to distract them from attacks.
Puffy uniforms, feathers on helmets and so on could lead the enemy to attack them instead the body of the soldier.
There is a reason why AoE2DE has a popular cube mod. It removes all clutter from the background turning resources into simple cubes for very easy readability.