Summing Monthly Numbers with Increases after Specified Intervals

I am trying to project out monthly amounts, with the amounts for each individual month able to increase by a certain amount after a certain number of months.

I can do this with a waterfall (Line 20 of the attachment) but, given the size of the model, I’d like to avoid that.

I am not super-knowledgeable with this stuff, but I can get close by using a sumproduct (Line 23). But it’s not exactly correct because for each individual month, the number of months thereafter that triggers the increase starts counting from the very first month instead of the month which contains the original amount. I hope that makes sense.

Any help would be appreciated!

Thanks