So about synthetic crystals and the Rogue One: Catalyst novel...
I'm wondering if one of the major reasons the Empire failed at creating synthetic kyber crystals was a combination of no Force users involved in the process and simply being too ambitious and trying to create larger-than-typical kyber crystals.
Going from Legends lore, the process of forging a synthetic lightsaber crystal was a highly-involved process that took intense focus/concentration as the Force user (generally a Sith, but some Jedi such as Luke used them too) had to literally manipulate the forming crystal at the molecular level to get it to grow in the proper way, which probably also allowed them to infuse the artificial crystal with Force energy, thus making it suitable for use in a lightsaber.
Sidious probably has the process of creating synthetic crystals down to a science, but he's not going to share that lore with someone like Krennic anytime soon; better to withhold the knowledge and see what solutions the ambitious bureaucrat comes up with. Maul certainly knows the process, but he's kind of focused on staying off the radar (assuming he's even still alive by the time Rogue One/ANH occurs) and not exactly the trusting type, especially where the Empire's concerned. It's unknown if Vader knows the process (some Legends material suggests that Sidoius gave him the synthetic crystal that he used in his new lightsaber after his reconstructive surgeries), but he's even less likely to share that knowledge than Palpatine.