So I was playing BF last night in a 4v4. I actually held my own booming my eco correctly for the first time in 20 years.
Problem?
Late Game battles about an hour in. Lost the match but I have a question. What is the best plan for late game battles? Send Wood/food units first and save the heavy hitters with gold later?
That sounds like a plan
What was your civ?
For a late game BF game you actually want all the best so trash units discarded, ideally, you set up a trade line
edit: and if for example your team lacks siege ram and enemy has them, you probably want to skip ram tech if possible and push early (rush early, a quick boom -dont make 5 town center boom, 11-, etc) or even sling (give resources to an ally so its faster), because if enemy is able to research siege ram (more hp, more damage) the outcome is obvious, just an example.
You can go full trash (skirm line, pike, light cav) with siege too if you know you kill (counter) enemy fast like that. There is a lot about BF and aoe, guess the short answer is, it depends.
500 steppe imbalancers 
Lithuanians. I really like them for some reason.
yeah as boat said previosuly you want trade for sure (like 20 trade cards)so you dont have to only spam trash. in general i spam trash as meat shields (whichever is best) and then use unique units/CA etc to do damage… imho rams are very poor in BF since they can be blocked so easily and meleed… i think rams in general are very poor in trash wars since your gold is tied up into something SLOW that has lots of hard counters
i prefer sinking gold (in trash wars) into tribs/mobile damage dealer/soaker. im enjoying incas atm with their huskarl level pierce armour unique spearmen to do the damage to the enemy trash units
also found even basic onagers super useful against non britons
This guy is right. BF is a different ball game. Trade lines are essential. Team bonuses and civ choices are even more important. In a 4v4 having a diverse set of civs is key to victory. One civ should be a tank civ with teutonic knights or Franks paladins or elephants. One civ should spam infantry and the other calvary. Lastly support (ranged units).
How many trade carts should I have in a 200 pop game on BF?
Well it depends on the situation and the civilization or the units you making. For example, with 30 carts goths halb + husk spam could be more than enough, 50 carts for a paladin civ like franks, 100 (!) for turks bombard towers; stone market prize can sky rocket. (numbers with no spanish trade)
We can put it other way, when to stop making villagers? when you have plenty of resources at 200/200 and your army feels small. When to stop making carts? when you and your team are reach enough to maintain the army and win the game.
Thats my huge weakness. I never focus on trade carts. I feel like they take anyway too much from my population
Well, you can delete your former gold vills as you add new carts.