I do want to keep the combined-research approach because I think it effectively suppresses "runaway" games. I'm just wondering if it's worth the effort to redesign it to a lump sum.
The difficulty is we get no information for mods about when the agreement ends. To do a lump sum we have to check every turn's trade deals, compare it to the last turn, and check for things like if the player died, went to war, and so on... storing all these things in the savegame file. I haven't done it before it's probably ~ten hours of work for a gameplay effect that's not very different. However, I'm revising a lot of the internal code of the mod lately to make stuff like this easier, so I'm revisiting this idea.