The answer to all of those questions is, more or less, cranking up DISCOURSE_SIDEKIQ_WORKERS.
I would crank that up to maybe 32 since you know you have a lot of spare CPU available. If you still have lots of CPU available after that’s been running for a while, feel free to crank it up more.
You could probably drop that back down to, say, 8 or 12 for normal operation.
Make sure you have enough max_connections for postgres. You’ve probably already bumped it up since you’re running multisite, but keep an eye on it.