However one issue I still have is that with RBM there is no good niche for lightly armoured troops, you basically just want elite units with heavy armour. The heavy armour doesn't slow down or tire units enough that lightly armoured troops can kite or outmaneuver them which should be the natural answer to them.
Is that really a bad thing though?
You spend quite a bit of time and money on investing in high-tier armored troops (or armor for yourself). They
should be better overall in a fight than lightly armored troops. Both from a realism perspective and a gameplay perspective.
Armor slowed those wearing it down, but not massively. Mail, depending on the coverage, could weigh 7kg-15kg. A full suit of lamellar or coat-of-plates was 15-20kg. When evenly distributed across the body, this was not actually
that cumbersome. Watch this video of a 20-30kg suit of full plate for comparison.
Lightly armored troops "kiting" were not the "natural" answer to heavily armored troops in real life. They ran away not because it made them more effective, but because they had to: they would lose in a straight fight to an armored man. They would choose to be armored themselves, if not for the cost.
I miss the stamina system from Brytenwalda. I hope in the future there will be some kind of fatigue system
Don't get your hopes up for that in vanilla. While personally I kind of like stamina systems too, a LOT of veteran M&B players are VERY strongly opposed to the concept. On top of that, TW seems reluctant to introduce any kind of new mechanic, let alone controversial ones. Of course, modders can introduce it easily enough.
Just in general I think a lot needs to be done about how different units have their optimal use so the endgame of combat is not just "get the strongest units and they beat everything."
That could be solved by changing the designs of the different types of unit, to create clear advantages against other types of unit;
soft counters.
For example, pikemen are strong against cavalry (something that is going to happen when TW adds pike bracing). Two-hander infantry are strong against infantry who have large shields. Large shield infantry are strong against ranged infantry.
Ideally, these soft-counters would be balanced so that T4 units can beat the unit type they counter even if it's T5, and T3 units can go even with T5 units that they counter.
That way, simply getting your units to T5 won't be enough to win every battle. You'll also need to know which units work well against which other units, and commit them appropriately, or else you could lose your high-tier knights by charging them into the enemy's mid-tier pikemen.
I also think party size should be limited by unit value rather than numbers, make the player choose between numbers or quality. It would give us incentive to use lower tier troops
Players already get two big incentives to use lower tier troops, which are: "Higher tier troops have significantly more upkeep" and "I literally can't use higher tier troops yet because they haven't got enough XP". The player spends plenty of time using low tier troops.
Does the game
really need to make everything a perfectly equal choice? I appreciate the importance of balance but this is more a question of progression than balance. It's kind of like you're saying that there should be more incentive for the player to use their starting horse instead of a Pureblood.
I agree with Apocal's post. Nerfing armor with the goal of balancing it relative to unarmored units will be counterproductive to making unit tier more important.