It's very easy to adjust tax if you're using the module system.
Search for the following code in simple triggers
(try_begin),
(party_slot_eq, ":center_no", slot_village_state, svs_normal),
(assign, ":cur_rents", village tax),
(try_end),
(else_try),
(party_slot_eq, ":center_no", slot_party_type, spt_castle),
(assign, ":cur_rents", castle tax),
(else_try),
(party_slot_eq, ":center_no", slot_party_type, spt_town),
(assign, ":cur_rents", town tax),
(try_end),
If you scroll down a bit you can also change the prosperity and campaign ai multipliers for tax.