It's doable with lua. It is one thing that's actually still pretty straightforward with civ6 available lua.
Not sure whether the modifiers system would handle something like that, but I doubt. I think the issue would be the repeats.
I might be able to look at the exact code specifics needed tomorrow or Saturday (I've already got a Really Advanced Start "Lite" mod I made for easing mod testing that creates units). If done entirely through lua on an automatic EveryX do Y basis, I would need specifics. Once every X turns, once every turn, one new unit Y for every unit X every turn, or whatever. And as you stated in the OP you'd want Unit_X to create Unit_Y, but never additional copies of Unit_X, because this might cause infinite unit spam.