Mites aren't really visible except under microscope. So if your bun is having dandruff and losing fur, fur mites are a good possibility. Fur mites will be uncomfortable and cause itching and fur loss, but shouldn't be initially harmful unless it gets to be a more severe case. So if you aren't sure it could be this at this point, you could wait and see if that's what you want to do. But definitely if it worsens, I would recommend getting it checked by a knowledgeable rabbit vet.
With flystrike, there will be some sort of open wound, though this can be hidden under the fur if a careful examination isn't done. Flystrike is maggots getting into the skin, so completely different than dandruffy skin. Outdoor rabbits will be more at risk, but it can still occur with indoor rabbits, in warm weather when flies are present. The rabbits at highest risk are ones that have damp fur, such as from a urine soaked bum or consistently wet chin, mushy poop stuck in the fur, and/or if there is an open wound on the skin. Rabbits with clean dry fur and no wounds, are at very minimal risk, particularly when indoors.
Unlike with fur mites, flystrike is an immediate emergency. If swift quick action isn't taken, chances are a rabbit won't survive. That's how quickly it can progress. Usually the initial signs may be some restlessness and may show irritation at the spot on their skin. Then it can progress to lethargy and weakness, then paralysis. In a matter of hours it can become fatal. So prompt action is always necessary.
https://wabbitwiki.com/wiki/Flystrike
To me, the dusty fur would point more to either dirt in the fur or fur mites. But if you aren't sure and are still concerned, it's always best to consult with a knowledgeable rabbit vet.
https://rabbit.org/vet-listings/