I like the idea of diplomacy points in principle, even if I do not use diplomacy in its current state, but am wondering about one small aspect of it: spending diplomacy points to end wars. In other 4X games I know, bargaining for peace usually entails signing a treaty which prevents hostilities for at least X turns (10 seems quite standard), and for good reason. Apparently, not so in SiS. In SiS, spending points on peace seems absolutely moot because peace lasts less than one turn: the AI can declare war on you again at the end of the turn. Not only does this feel bizarre, it deteriorates the functionality of the bargain to the point of uselessness. Could you consider doing as others do and add an X-turned-length treaty aspect whereby X is larger than 1?

I don't think of it that way. I don't believe in the idea of buying peace unconditionally with diplomacy points. It would be too OP.

Instead if you think the peace isn't going to last long, maybe it's not a great use of your diplomacy points.

However if that's how it's meant to be, then this is probably the symptom of a UI issue. If the AI is really hostile to you to the point of declaring war again, then the UI should warn you or disable that option.