Honestly not sure why you are having a problem. Can't remember last failed synth whether leve or grind and I'm synthing things at least 5 lvls above my lvl (29 CUL).
Deciding whether to use main or off-hand is simply which has better overall stats (unless going for HQ).
My equip is all below lvl 20except lvl 23 Main hand and lvl 20 Off-hand. Equip focuses on Control and Craftsmanship.
I always users Master facilities support (If you're doing leves then this is easily affordable).
I use Orange Juice (made myself) to boost Magic Craftsmanship
From the beginning I use Maker's Mark first and use STandard until Maker's Mark runs out.
I then use Fulfillment.
I keep using Standard until around 70-80% Complete. I then use careful until I hit around 20 Durability. AT this pint if I'm at 92% or higher I simply use two more careful. Anything below 92% I use standard and am guaranteed a completion.

I do have Preserve equipped as backup.
I also have Creative Urge equipped and sometimes use this on final synth if slightly nervous about getting completion.
Even without rested exp I get around 1k per synth (Approx 150+ Quality)
Can also get around 900-1k exp on lower level synths by starting Careful synthing slightly earlier and aiming for 300+ Quality

Hope this helps