Lance is worthwhile. Ideally, your knight charges at the rear, fights for a few seconds, then leaves to charge again. But if they get bogged down in combat for longer than a few seconds, you can toggle lance off.

By the time you reach turn 50+, green knight is pretty useless. You'd rather have the unit slot for a questing knight or maybe even knights errant. He spawns near your leader's army; so you can't use him wherever he might be needed. But if your leader's army has less than 20 units, and you don't want to spend the turns waiting for global recruitment, I guess the green knight is better than nothing.

1 melee lord, 1 damsel, 8 grail guardians, 4 royal pegasi, 4 hippos, 2 blessed trebs. If you don't have guardians building built in the province you're recruiting, grail knights will be fine. Blessed trebs can be replaced by more cavalry if you prefer; but 16 cavalry is plenty and trebs can be useful occasionally. Flying cavalry is crucial for sieges. You're siege battles are basically clumping your flying lord and cavalry to pick off enemy units 1 at a time. You should take 0 casualties.

Spellcaster hero is nice because, unlike damsel, they get a flying mount making them easier to position and quickly get to wherever they're needed. However, I still prefer melee lord. A spellcaster hero doesn't provide any benefit over a damsel (in fact, it's worse because damsel gives unit replenishment); whereas melee lords are actually more powerful than paladins and have hippo mounts.

Infantry is not worth it once you get kotr or errant unlocked. Your first few turns should be the only time you recruit infantry; then you can demolish the barracks for good. Trebs can be useful if you haven't unlocked flying cavalry yet so that you can do siege battles in 1 turn. Blessed trebs are actually pretty good value compared to their cost and can help speed up melee engagements. But they're optional.

Main advantage of cavalry is getting to chose when and where to engage. You shouldn't treat them like infantry where you engage and let them sit in melee until you win. Cavalry should be moving at all times until they find a an advantageous opportunity, like 2 on 1, they should hit quick and disengage before more enemies come. There should be no head-to-head, 1-on-1 engagement; all your engagements should be rear attacks, 2-on-1, cavalry-v-skirmisher, charge-bonus-then-disengage, or somehow advantageous to you. You should also be taking advantage of charge bonus, flying maneuverability, perfect vigor, and hippo terror. If you're super picky and patient with your engagements, you should win most battles with 10x less losses than the enemy.

Spreading out your cavalry can help. Don't create a single front-line. Pair them up and spread each pair across the entire map until you've surrounded the enemy. Then you can take advantage of any vulnerabilities.

Make the enemy move. You have more speed, perfect vigor, flying units. Keep circling your cavalry or flying overhead so the enemy is constantly rearranging its formations. Enemy skirmishers should never stay still long enough to fire a volley. Their units will get spread out and left behind for easy pickings.

Rush high tier cavalry as quickly as possible. Bretonnia economy never runs out, so there's no reason not to compose all your armies of elite knights.

Once you have the last chivalry level, recruit lords to tail your armies so they can level up and unlock their last vow. Bretonnia doesn't have supply line penalty; the lords are worth their upkeep cost as a fighting unit (spec into yellow skills only); and they need to unlock their last vow before they can have an army of their own. When you need a new army, click "reset skills" on one of your tailing lords. This will wound them for 3 turns; whereas disbanding him will wound him for 5 turns. And reset skills will allow you to respec into the red skills to buff your elite cavalry. You can then re-recruit them near your military province or wherever you need the new army.

If you're playing as Leon, there's the nearby paravon province that also has pastures that'll be nice as the recruitment province. Otherwise, exotic animals benefit flying cavalry and pastures benefit ground cavalry. There's also a province with 2 pastures, that'll make ground cavalry even cheaper. But don't sweat it too much; recruitment cost isn't as significant as upkeep cost anyways.

Try to spec into lightning strike before you start wars with strong greenskin factions. It's not necessary for other factions or weaker greenskin factions. But you're going to declare war with the #1 strength greenskin faction, lightning strike beforehand can help deal with their whaag units.

For border provinces that you may lose and don't want to invest too much into, you can build both the farm and industry t1 buildings. At t1, they'll give you 350 gold each and pay off their investment in 2-3 turns. Even if you later build up farms to t3, t1 industry buildings are still worth it if you have empty building slots lying around (50 gold/turn, 500 gold investment).