So basically this mod now allows loaders to unload onto a belt with stacked items before any belt stacking bonuses are even researched, which wasn't the case before. Is this intended?

Edit: Now that I look at it, there is an unannounced +3 belt size bonus from nowhere in my save now. Wasn't the case last time I played.

After loading just Space Age and this mod, it seems there's now a passive +3 belt size bonus.

the stacking bonus is immediately learned for the amount selected in the loader stacking size setting

So the only two options are to either have free additional stacking or not having loader stacking at all?

I can do this as a setting

is it possible to have it increase with the research?

Yes, I can add this feature.