A few tips:
- Drop your party into the garrison just before payday (happens every 7 days, in the morning) to halve their wages
- Bet and win tournaments you come across
- Get 10 Prisoner Management asap for maximum amount of prisoners takable and for highest chance to capture unique spawns
- Get troops with blunt weapons for prisoner hunting: all your companions, the whole Inquisition troop tree, Sarleon Recruit, Sarleon Cavalry, Sarleon-Ravenstern-Pendor Man-at-Arms, Ravenstern-Pendor Knight, Sheriff, Mystmountain Shaman, Mettenheim Regular Infantry, Eventide-Ebony Gauntlet-Radiant Cross knight and sergeant, and a few more.
- Tell lords to go into one of your walled fiefs, or call for feasts often as king/queen. The lords coming in will leave their prisoners in your garrison. Sell them for profit.
- Capture and ransom the enemy lords (and kings) that you don't want to have in your future kingdom (i.e. the ones with bad personality such as bad-tempered, sadistic, pitiless and cunning. Although the latter type is manageable after he gets 3 fiefs)
- Get high looting skill, fight, and sell the loot
- Optimize your garrisons: maximum 300 top-tier (lvl 26 < x < lvl 36) factional troops on foot (not mercenaries), or 500 medium-tier (lvl 15 <= x <= lvl 26 troops for towns. Half of that for castles
- Or, have no garrisons at all, and focus on attacking instead of defending your fiefs. After all, if you take fiefs faster than you lose them (Engineeer 10+4 ftw), then you're golden and you'll win the war
- Get 10 Leadership asap to reduce your wages (-50% with 10 Leadership)
- Buy an enterprise in every town (you already did that)
- Befriend minor bandit factions (help them in fights) in order to greatly lessen the attack on your peasants and trade caravans. That way, they will almost always reach their destinations safe and sound, increasing prosperity and your fief income eventually.
- Don't get any tax inefficiency: maximum 4 towns, or 3 towns and 2 castles/villages on good campaign AI
- Or, f*ck tax inefficiency with an overdose of opulent fiefs.
- Speaking of which, I recommend you to try this out once:
https://forums.taleworlds.com/index.php?topic=326944.0