My last mini-campaign ended around 8th level. The main villain was Harrumah the Long-Beard, an aged dwarf lord who was holed up in his fortress issuing demands to a xenophobic dwarven kingdom. The group eventually found that his fortress had been sealed from within for centuries and that the dwarf lord had actually calcified upon his throne over the generations. So they were hacking at a statue that was possessing the other members of the party and sending out ancestral wraiths to fight them.
I no longer run 5e "by the book," so my stats probably won't be much use for the OP. I do give my villains (even "mini-bosses") special abilities: auras, legendary actions, the ability to push/pull/slide/immobilize characters, etc. Just adjusting the numbers wasn't doing the trick for making them interesting or challenging.
If anyone is interested in the math or more specific tips, I'll provide them. Just don't want to derail the thread to much.