I get a few work units, then it goes dry so I allow in some other work to keep the machines busy. Then some HST work comes my way, I turn off other work in the hope of a steady stream of work.... rinse, repeat....

I only have 5 HSTB tasks in progress atm. I was running about 50/50 split over 64 threads with FAAH2 for the last week or two.

In device profiles, limit units for other projects so that your threads have work without completely fill the cache.
Then put the maximum for HST and fill the cache.

If units for HST are available, you will receive.
If none is available, you will calculate for the project(s) of your choice.

That's what I did before reaching my goal of 2 years.

My HSTB queue is full.

On average have 8 tasks in queue and doing about 80 hours a day, letting a max of 4 process concurrently. The device runs at low frequency allowing to keep power consumption substantially reduced and run 24/7, stretching the jobs quite a bit... 20-21 hours a piece of the 300-400 series momentarily. Most of the jobs seem to arrive in the first half of the day, UTC.