The last comment in the other thread mentioned they needed to force FOREMAN_PUMA_WORKERS to be greater than 1. You currently have -1 in there.
If I’m reading the facts you provided right, then that machine has 4CPU cores and ~8GB of ram. Considering you want to do repo discovery I’d guess you also have katello installed. The hardware requirements for foreman with katello are much higher than what you currently have there.