I haven’t read Catalyst either, just a warning, but I know that it spans from 22-17 BBY, and that during that time kyber crystals had already been acquired from some other place. Galen Erso’s research was largely completed by the end, I believe, so once the events of the book were over I think it’s possible that the Empire could have started mining as early as that same year. On the other hand, the prologue of Rogue One is set in 13 BBY and the rest in 0 BBY, so that might be just too early for the mining to be finished when it was. It seems to me that anything later than 17 BBY and earlier than ~10 BBY could be plausible, and that something close to 13 BBY (when Galen was recaptured) could be more probable. That’s my opinion, though, and it’s really up to you.
Anyone who has read Catalyst and could provide some information on this, please share!
~ Jacen