(does CK actually calculate tax income feudally, i.e. you get % from nobles refracted by loyalty etc., or is it calculated raw from nation population?)
Multiple stages, you first have the province's base tax income from holdings, buildings, and modifiers, which is then changed by the Stewardship of the people directly in control of the holdings, then the congaline of % tax up the feudal ladder modified by opinions starts.
? I wouldn't care if it was just about them "not liking" - this is about them not paying what they are obligated to, which was never trivial.
What I'd do is give an uppity vassal a decision: a) to pay in full or b) withhold a certain amount, which then gives their liege a casus belli to enforce the full payment for 10 years.
For balance reasons, I'd make using the CB add a cumulative "tyrannical ruler" modifier to all vassals, but the CB should be there - it makes zero sense not to. We aren't playing Crusader Cucks here... are we?
See, being able to use a CB to force full payment of taxes would be TOO EASY. It'd make managing your finances trivial. Furthermore, it'd be a completely gamist mechanic.
CK2 uses an abstraction of medieval taxation to make it into something simple enough to be used as a game mechanic (ie, taxes are collected as cash transfers to royal bank account, rather than all sorts of shit whatever peasant happens to be able to scrounge up as a living in a subsistence agriculture and almost never in gold or currency). Basically the gist is that while you have a GUI telling you that your vassal isn't paying as much as they could, the actual ruler you are playing as would have absolutely no idea this was the case. This is essentially an abstract form of corruption that's baked into backroom politics. The low opinion vassal is not merely someone who doesn't like you, they are a potential rebel and most likely a part of one dangerous faction if not the leader of it. They'd have every reason to come up with whatever plausible bullshit excuse (see letter saying "doge ateth thine taxes, liege, the peasants art starving woe is us" or whatever) but ultimately the game aspect here is that you are presented with a problem that if left unattended will at minimum sap your resources to grow as a problem or will explode into a bigger problem, with now you having to find a way to deal with the problem of which the game presents several. The trick is in finding a way that doesn't cause tyranny. Another thing to note is that the tax system itself is designed to work as a way of tying into the game's feudal state management mechanics, historical simulation would just have no taxation at all from the nobility (one of their most zealously guarded privileges until the downfall of estates system all across Europe), and would really just steal from your shit to build golden outhouses and such bullshit.
Also of note regarding earlier mention of 50% taxation is that you have been too inundated with living in the time of sensible and relatively human governing and economics. 50% was the low end of tormenting your peasantry scale, in places like Sweden-Finland it's estimated that it might be as high as 75% (EDIT: Of further note in regards to these estimates is that income taxation or percentile based taxation was not a thing, most common form of taxation was an arbitrary amount of tax based on what someone thought the peasant should be able to produce on their patch; we also aren't getting into all the exemptions and special cases like equipping and supporting a horseman for the king's army to gain tax exemption in Sweden-Finland). Thing is, taxation was not exactly an all that organized business, and one reason for why France organized itself so rapidly after the triple Henry shenanigans was that the estates of each subdivision of France handed the king indefinate right to set whatever new bullshit taxes they could come up with. That's also a thing that's not really presented in the game, that taxes consisted of many many bullshit taxes that were especially during wartime made up very arbitrarily to squeeze out every scrap of loose resources (mainly agricultural products, naturally) that could then be turned to cash. Another example of abstraction is that the Church takes its tenths from you rather than from all your peasants. If the game had a realistic presentation of medieval taxation, most players would be very confused by it, in rising debt funded by Italian bankers, and thinking "oh god I don't know how these poor peasants put up with all this shit heaped on them."
