I agree for that peasants spam isn't good idea. I think peasnts should cost 200 and they most be 2lvl to get that horeses. Theoden's leadership should give extra xp bonus and his skill Times of Glory (5 or 6 or 7 lvl skill) should be changed into some healing.
Théoden already has a skill to increase experience gain, called Hour of Glory, and both Elfhelm and Hama give passive healing to units. I think that adding a requirement to have the peasants be level 2 is a good idea.
That "peasant/bowmen to cavalry" stuff could have some limitations, I am thinking about a final power, once activated, for 1 minute the stud farm will automatically transform any infantry units near it to cavalry units(lv 1 no upgrade), however, in this period the stud farm will generate no resources
Would this REALLY be worth it as a 10 point power? Compare it to the current 10 point power, which by itself can generate a full, free, permanent standing army of fully upgraded Rohirrim. It needs to be comparable to that.
Alright hoho here is my rebuttal. My idea to balance the 10 point power. Say that the level 10 power transforms all to cavalry, but a weaker form of cavalry. Call them something like "Peasant Horsemen" and make them like half as effective as Rohirrim, but still dangerous in numbers, kind of like the traitors on horses, but can receive upgrades and level up.
Right now, against an Isengard pike/shieldbearer spam or a Gondor Tower Guard spam, Rohan will lose. If you don't believe me we can play and you can try it, the only place Rohan has a chance is early and mid game. Late game, once the enemy has their heroes and fully upgraded pikes, you can forget about killing them with cavalry.
For the Assembly Point bonus, it wouldn't necessarily need to be so strong, it could be something like 4 of them gives a 30% boost. But seriously, just compare this to other factions:
Rohan gets 1 SINGLE special discount on cavalry. That's it.
Gondor gets a discount on cavalry, upgrades, AND elite units, and a power recharge time decrease bonus from their buildings. Also their stoneworkers can give them discounted buildings.
Mordor gets discounts on trolls, all orcs, power recharge times, and upgrades.
Isengard gets an incredible building discount from Lumber Mills, a discount on units and upgrades, and their outpost makes their upgrades insanely strong. Pikes with those upgrade buffs and shieldbearer/captain combo completely demolish Rohirrim, no contest. After directly comparing the factions you can see the gap my suggestions are trying to fill. You also are not taking into account that building 4 assembly points requires 4 settlement spots, and they don't generate resources, and they cost 850 to get fully armed and operation. That's a NASTY hit on economy, but the building speed improvement combined with the healing effect of the assembly point would give some interesting strategies to a skilled player.
Also, you say since when has Rohan been known for huge peasant armies, and for making huge armies quickly. I draw your attention to both the movie and book, in both the army starts out as a ground force of militia, who use a shield wall tactic. Once Théoden calls the Muster of the Rohirrim, everybody who can bear arms comes to dunharrow 6000 troops in only 2 DAYS. All of those infantry then mount horses, and become the mighty force we know and love from the Pelennor. So this would represent them perfectly, doing a peasant spam and then using the 10 point power to make them all mounted is an identical representation of what happens in both the film and the book. Heck, you could even call the 10 point power "Muster the Rohirrim!"
For the Farm bonus, firstly I'll address that some players don't do all outer farms and spam peasants, but do outer stud farms for the discount on cavalry. Me personally, I usually only do 1 single outer Peasant Farm near my base so I can use corrupted Théoden's taxes power on it for the early game money, and I use a few battalions of Traitors to help me hold the line. Giving all Infantry a discount would add a serious incentive to have an infantry build for the game, not to mention making it worth it at all to make archers. It would also mesh very nicely with Gamling's late game skill Rapid Deployment Order, which right now probably doesn't get used a whole lot, because like you said late game Rohan only has cavalry, for now anyways. Also, with this idea an army of fully upgraded peasants and archers wouldn't really be "a lame farmer army" it would be a force to be reckoned with, used properly with heroes. I think it would be good for the forces of light to have a spam faction as well, it would add some great diversity to the gameplay.
So that's my rebuttal. REALLY think about it everything I've mentioned here and the effects it would have on the gameplay, and you'll see that none of this would really be that overpowered. Rohan only gets 7 build plots in their castle, you need 1 for stables, 1 for armory, and usually 1 for a well, leaving only 4 inner build plots for stud farms or the recruitment camp/training grounds/whatever you want to call it building that I suggested. On most maps holding 6 outer farms against a player who can harass decently is no small feat. So getting any of these bonuses is not gonna be easy, same as any other faction. It would just give the player some options for different playstyles, rather than just early peasant spam followed by all cavalry, or an all cavalry rush. Rohan has no infantry option right now mid to late game, not a serious one anyways, and this would cement them as having the 2nd weakest, but also 2nd cheapest infantry. The only army they have a chance to beat late game is Mordor, but even Mordor can just do a pike spam combined with the debuffs of the Nazgul and murder them. Rohirrim without leadership don't stand a chance once their Glorious Charge runs out. Also, instead of just bashing me
come up with some bonuses of your own! Debate is great, but only if it produces good ideas
All of the ideas I gave could of course have their effects nerfed a bit too, I just used up to 50% as an example. I'm awaiting your reply
EDIT: I also have another suggestion, lower the standard Rohirrim units CP to 60 like infantry.