It's pretty widely accepted though. He himself hated the idea so you can expect he did the calculations thoroughly.

But you want that to happen in space and to control the output of energy.

Otherwise you just have a bomb.

In the same way that atomic weapons and radioisotope generators both convert mass into energy. It's just a matter of slightly different timescales.

How could we harness this energy and make it usable?

The biggest problem is that if you're creating it with lasers, you're only going to get the energy out that you put in. You really want to be able to feed it matter, which would effectively make it an anything-to-gamma-radiation converter, which means you have to feed it quite a lot of matter, against the radiation pressure of all that energy coming out. The paper mentioned assumes a worst case of not being able to feed the black hole at all, but doesn't (in my skim) address the fact that this means you have to put in all the energy you'll be using for the lifetime of the black hole at the creation of it, which seems significantly more outrageously infeasible than the bare necessity of creating a black hole at all.

There’s a recent paper on the formation of such a “kugelblitz”; it’s argued to be unfeasible.

https://arxiv.org/abs/2405.02389

Probably more like a water wheel - matter spinning around the hole can be accelerated.