I agree that the setting should imprint this on the players, which is why I said I do not advocate for a 1:1 conversion of the original 5e usage. The inherent problem here for me isn't limitations compared to the original, I just feel like the limitations go too far and are uneven between approaches to the point where some are borderline unviable by comparison if you want to have at least a few options beyond just cantrips over the course of a in-game day of play. Having no spell slots is a good idea for the conversion, but overly limiting the core functionality (and the core identity of play) for some classes and even having it be quite different within the same class based on approaches is to me taking the danger-aspect a bit too far. As much as a Dark Fantasy world should come with its risks, end of the day I assume most people play to enjoy themselves, feel useful and in that have fun.My thoughts on this is that this really catches the feeling that high level magic is dangerous. It should also be mentioned that just because you have enough temporary Corruption to force you to roll for a mark of Corruption, this doesn't stop you from casting more spells. In a way you have unlimited spell slots of every level, but are you prepared to pay the price for it? Getting permanent Corruption is on average a slow accumulation over several game sessions.
Does this mean that martial classes are better than caster classes at levels 17+? Perhaps, but looking around different forums and subreddits is seems like the general consensus is that the caster classes vastly outperform the martial classes in standard D&D 5e at higher levels, so perhaps this would end up being more balanced, even though with some imbalances still.
Totally agree with you here, this is a real corker of a thing to try and hash out just purely through theoreticals. The problem is though from even these 'what ifs-' and theoretical frameworks the threshold doesn't take away from the double usage of corruption where one hampers the other and vice versa. I think the idea of a one time cost for unfettered use is an interesting idea, but ritualistic use is still something that carries a high tax (it is permanent instead of temporary and as such it cannot be shed as easily or at least clawed back a bit) and it effectively reduces your pool "permanently" making the free cast a smaller comfort than it might first seem.Yes, it's hard to get a good overview of this without detailed play testing. The casters do get a Corruption threshold that is much higher than the martial classes. And learning rituals gives you permanent Corruption when you learn them, but no temporary Corruption when you use them. It's hard to evaluate the exact game balance with all these different interactions.
I am not saying that this is an easy issue for the lovely people in charge of making all this work to solve, but I do worry about these things in particular and as such I rather voice my concerns than grumble in silence and shrug at my players.