I also agree that the materia meld rate shouldn't be touched since it's the balancing mechanism of the system and raising the odds even slightly would throw everything out of whack. 1% more successful melds adds up to a ton more multi-melded materia in existence. Having a high craft level is already required to perform melds of any sort of relevance already. Once the HQ system gets revamped and you're able to better prepare/aim for it, you can use your high tier crafting gear and high crafting level to aim for that and that in turn will increase your success rate.