You just hard cast Ninjutsus, and use the dot during Trick Attack, past that it's more or less the same class as it was in 5.0. If you mean you haven't played since 4.x though, that's a whole bigger issue.
https://docs.google.com/document/d/1otFJjmazk1z55gbcynSHQyXOzC2IvnNUM_q5KwVWvvg/preview This is the balance guide, I'm not sure how accurately updated it is for the current patch, but it should give you an idea of the 5.x playstyle and the new additions. If you meant 5.1 honestly just hit a dummy for a little bit and you'll see it's very much the same as it was before but instead of doing shadow fang combos you spend those GCDs on doing Ninjutsus and that's really the main difference(well cooldowns line up better too, no more Meisui calculations since it's always up with TCJ for example).
Class is actually absurd for leveling/soloing, been doing my beast tribes and hunt bounties with NIN and it's hilarious how strong it is at bursting down solo mobs when you can reset both charges of ninjutsus inbetween every fight. Gameplay reminds me of that old LARP video with the kid yelling Lightning Bolt Lightning Bolt, it's just Raiton Raiton Kassatsu Raiton(no 76) and stuff is close to dead, then next one TCJ Shuriken Raiton Suiton Raiton Raiton, don't end up meleeing all that much.
As for your crafting question, not sure what you mean Rapid Synth was never 100% even with SH2, it was 90% and was basically something you never used unless it was a hailmary or you were trying to, well, rapid synth items like for Namazu quests and what not. The main strategy, before and after the change, is to use Delicate Synthesis(100% synth and touch), only now you slip ingenuity and innovation before your delicates to get better progress, and then you finish with the usual great strides byregot and a careful or basic synth to finish depending on values. You start with Reflect(gives Inner Quiet and +2 stacks for only 24CP) and use Manip and/or Waste Not II to keep durability in check, after that it's fine tuning depending on stats and items created, with preparatory or prudent touches as needed to build stacks without moving progress, but this is the basics. It's honestly very similar to prepatch, just with less filler stuff like Steady Hands and Comfort Zone(although arguably the new innovation kinda does that).