With TweakMB you can putz with tax inefficiency, base income levels of fiefs, and an overall multiplier of tax rents under "Budget Management". Not sure if that's what you're after, though.
I think that's not what he's after, since TweakMB IS (or rather does) a MODification.
However, I don't wanna be a smartass so I have to tell you, Orety, you've no chance to fight tax inefficiency by ingame means like visiting your fiefs more often or building improvements. You can rise the tax income of a fief by improving it (with a mill) and by protecting it from bandits or raiding parties, but as long as you don't have to collect your taxes personally, you are bound to bear the inefficiency.
This brings an idea up my mind and I have to switch over to the Diplomacy forum...
Build mills in villages. Try not to get your villages raided. Keep an eyes on bandit camp in your villages. Escort caravans. Discourage lengthy sieges, You get a 33% of -1 town prosperity per day it get sieged IIRC.
Basically, try and keep centre/village prosperity high.
Campaign Easy: 3% inefficiency for each center after your 6th. You start actually losing money at 23rd fief. Inefficiency cap at 28th fief. You start getting more money then at 22nd fief at 30th fief. Black zone: 23-29 fiefs
Campaign Medium: 4% inefficiency for each center after your 4th. You start actually losing money at 18th fief. Inefficiency cap at 21st fief. You start getting more money then at 19th fief at 22nd fief. Note that you do not gain any more money on average at 17th fief comparing to 16th due to tax inefficiency. If the 17th fief is a poor fief then you'd start losing money there. Black zone: 17/18-21 fiefs
Campaign Hard: 5% inefficiency for each center after your 2nd. You start actually losing money at 13th fief. Inefficiency cap at 15th fief. You start getting more money then at 12th fief at 17th fief. Black zone: 13-16 fiefs
Try not to grant yourself any more fiefs if the amount of fiefs you own would land in the black zone. Do note that higher campaigns also have penalty on the total fief income, so dont hope to get more money at any range by pusing onto higher difficulty.
I have checked the codes, and there is not much you can do about prosperity of your lands. nijis has once claimed that he doesn't the economy to be easily influenced by the player.
The best way to earn income is to build player enterprises in all possible towns.
Pity, that - from what you wrote - player has so small influence on tax acquisition. I should be able to hire some tax collectors, or send one of my NPCs... but It's good we have modules.