First, my background: I’m a mediocre guitarist, but this happens to hit on a strong point of mine.
Pinky “strength” basically comes down to just one thing: your body knowing when and how to relax and when and how to Go! In other words, practice accuracy without accumulating tension. You know how people without hands get absurdly good at using their toes as if they were fingers? It’s exactly that. Anyone who is basically healthy does not lack muscular power for their pinky; it’s only clumsiness and tension that makes things weird.
The tricky thing about muscular tension is that most of the time you will NOT feel when your body starts to habitually tense up. This is why so many people develop problems with their with back and shoulders. You want to actually use your hand to grab your muscles and feel for places that are trying to ball up. So before practicing your pinky exercises, and frequently during the practice time, grab your forearm (both the front and back! don’t forget the back!), curl and uncurl your pinky a few times, and massage the area that you feel the pinky activates. I would bet cash that you will see and feel an immediate obvious improvement in economy of motion if you do this.