There's also the Penrose process which is theoretically useful on spinning black holes. I also watched this in the past, which has some fun ideas (that are probably a little fanciful?) ▶ this video tl;dw: put a mirror around it and shine light in, get back lots of energy and/or blow up your whole solar system. I'm not sure if any of that would apply to a ~planet mass black hole spinning with ~planet sized angular momentum, or how solid the physics even is, but it sounds cool anyway.
in “What if Planet 9 is a Primordial Black Hole? (2019)” · View on HN