It is better than having a 2 stable production for every 1 stable.
Suppose generic civ 10 stables vs Cumans with 5 stables + SH.
In 15 seconds you have 5 Cuman Hussars whereas 0 generic Hussars. In 30 seconds, you have 10 Cuman Hussars and 10 generic Hussars.
What’s the point of getting this tech and making cav archers at double speed when your cav archers will be trash in Imperial?
You can do this kind of Hussar raid without this tech too… wood lategame to add more stables is not a problem. I get that half of your units technically come out faster but when you’re in trash wars this is not very significant.
Actually if this is the case, Indian infantry takes out Mayan eagles in exactly 4 hits (no overkill). 13 + 4 (Blacksmith) + 8 (bonus damage) attack vs 100 HP.
And didn’t have a noticeable effect on other units, but maybe against Viking infantry it can help.
And the problem is cumans have a weak as crap early game. So replacing a castle age unique tech isn’t going to help
None of your suggestions make sense expect 4.
vikings: remove free hand cart or it is %50 cheap
aztec: give back +5 
goths: deers gives %50 more resources, +20 pop on imperial age
bulgarians: can research technologies on krepost, reduce base rof of konniks (konniks p.o.s. without stirrups tech), optional: knights cost %20 less food
franks: heavy plow not free, remove crop rotation or castles %15 cheaper
incas: buildings cost -%10 stone, villagers benefit only from infantry armor upgrades. (a late game buff required but i can’t find a unique buff)
slavs: change age of techs, (they are most underrated less played civ actually, need a rework)
buff steppe lancers and longsword
btw cumans need buff %100
Also add a unit with 9999 damage and 9999 health and 9999 armor
11 but konnik attacking really really slow without stirrups. it is 2.4 when knights have 1.8 rof
They are also two units in 1, so yeah.
Bulgarians are fine. They were picked quite often in tournaments lately and they have some nice options. I don’t see a reason to buff aztecs even if they seem to be less op now, but they are still not weak. Vikings need early eco as they have nothing else carrying them on land maps until fully upgraded uu which is quite expensive to reach.
I would rather nerf them in water a bit (15% cheaper warships in feudal to 10%, and team bonus reduced as well to 10%).
1111111
They have literally the best eco in the game
Still not at the Chinese level
Viking Longships need to be less op, they can easily hit and run my whole fleet( of fireships), that is until late castle age and above when he cant micro due to the size. Also even if i get my fireships near him, HE just runs away near his castle
Also, should not fireships in a whole receive a little faster speed?
They have the best eco on nomad lol
They have the best eco, especially on nomad , you can just give up if he reaches imp quick
And what else except eco? Average archer, worst cav and infantry which is never viable until imp. And Persians will boom better than them on nomad.
Lategame the most important res is Population. Faster tt saves Pop. I really dont see why you try to downplay this.
Viking going to imperial age so early. It is why we need to remove free handcart, it is really op eco. Also Bulgarians not okay, like other low tier civs. I played with them 300+ game and it was a pain in the *ss against A,S tier civs. Don’t have any eco bonus or military bonus for castle and imperial age, but great civ for feudal agression. ( I m 1540 elo rightnow)
The discount bonus works very well for both castle and imperial age. It makes it quite easy to outtech your opponent, and there is no way in hell free militia upgrades, stirrup cav and needing only 650 food to get siege rams are no military bonus.