They found a temple similar to what you're talking about on Lothal, however it had already been pillaged by the time we are at in the RP. There are still crystals floating around but they are extremely rare.

For example, when Ezra finds his crystal, Kanan is extremely surprised and nervous because of how rare they are, and because the crystal called to him, almost as if it were a person.

So your idea could work, just know that the temple or cave on Jedha is heavily guarded and would be a tiny bit difficult to get into due to the Empire harvesting every ounce of kyber they can, albeit slowly.

Also, I will allow legacy crystals but sparingly. The kybers being rare and sacred is what makes them so special within the new canon. So your legacy crystals are fine.