I'm curious what useful applications we have for black holes of any size

There are actually a lot of theoretical applications.

Black holes are likely the most efficient form of power generation that we currently know of.

Black hole propulsion may well be the best form of interstellar starship propulsion.

Black holes may also be the ultimate computer.

If the universe continues based on it current understanding then eventually all the stars will be dead and the universe will be a dark place. The Black hole era will last trillions of time as long but may well be the golden age of civilization.

Each of these topics is a rabbit hole.

The theoretical limit of how efficiently you can turn energy into useful work depends on how cold your energy sink is. The coldest energy sink in an ever-expanding universe gets colder over time as everything spreads out and the fixed amount of energy occupies more and more space. So the hypothetical optimal end-state of an extremely old civilization is hibernating a bunch of very stable mass until the extreme far future, at which point they can drip-feed it into a black hole to do some extremely efficient computation.

It's assumed that little ones are the most efficient mass to energy converters, via Hawking radiation.

It's simple, just deflate spacetime!