I followed Lachlan's advice and increased the concentration of Ilfotec HC to 1+31. And yes indeed, the grain has gotten smoother, here's a few 5000px scans of HP5+ in Ilfotec HC 1+31
They were all host wide-open
@F/1.4 on Voigtlander Nokon Classic II 35mm f/1.4
But the biggest surprise was Rodinal. I have avoided this developer since the beginning, but I love how small and uniform its grain is. Actually, without knowing what the Internet had to say about Rodinal, I would have described it as "fine-grain developer" for HP5, look:
This is HP5+ in Rodinal 1+50. Look how tight the grain is, none of those little "worms" that often show up in scans. Gorgeous!
There are benefits of being stuck at home for a weekend
I am definitely introducing these "old tech" moody developers in addition to my usual Xtol routine.