Hi there!
So after I read through your post and tried to soak up your information I sat down and re-worked my whole approach on this.
At this stage I am mentally not ready to dive into cpp so I tried focusing on the idea of a “spawn manager” and on “precalculate and storage”
It made me learn how to cut tasks in smaller pieces, rethink my whole code writing approach since now it has to work on tick/timelines, and especially gave me much more insight about how powerful the hardware is by squeezing more and more loops into a single “tick”, to see whats possible, how its affects the calculation time but also the different fps.
Probably far away from your idea of the spawn manager but for me, comming from "oh i can only have 120 loops before the infinite loop error crashes on me, so thats my limit 
" to "wooow its so fast and I now can calculate hundreds of thousands more things now, infinite possibilities 
"
Bottom line, much appreciated your spend time answering me, lead to me learning a lot.