Main BiS for getting into raiding is pick the highest ilvl pieces, meld vit6 on right side and make sure to meld non spell speed dps6 melds into the remaining slots. Optimising on the micro level isn't really worth it when you're going to end up replacing and upgrading
Now with the Byakko vs crafted, the differences between are going to be very minor keeping into account the crit and dh on + overmelded stats vs the byakko stats, extra int and weapon damage
Keeping it simple, there's more wd, int and vit on the byakko. While RDM wants to minimise sps it is not a 0 value dps stat so people shouldn't treat it like the plague unless they are stacking so much their mana is barely lasting 5 minutes
Given that you already have the byakko, unless you have a ton of gil which I'd doubt given your being a newer player, don't worry too much about investing in a crafted rapier which will be more of a sidegrade
other crafted pieces like legs are solid but again gil and overmeld materia constraint consideration
Sigma normal gear is perfectly viable
The helm and hands are pretty much strict upgrades over their genji counterparts