You don't have to replicate all the insanity that goes on in a modern OS.

Lock threads to a particular core on creation, every core does a Round-robin over all threads. You're 50% there.

Have the core detect congestion, and move yeet threads to another random core when it's congested. You're 75% there.

Use three lists for priorities instead of one. You're 90% there.

Make up for the other 10% with sane design in other subsystems.