If your thumb is too slow you should consider learning to claw (right index on Y, middle on Z, ring on R, thumb on C-stick / B / A). I can't waveshine, SHDL, FJTL, SHFFL aerials quickly, etc. if I use a conventional grip. I mean I can play Marth or CF with normal grip but Fox? Forget about it.
Clawing I can do pretty much every tech there is with good consistency. My SHFFLs are fast and clean, my waveshines are fast, I can drillshine infinite (cpus anyway), I can do reverse SHDL, shine stall on the edge, shineturned bair, etc.
It's so much easier it's ridiculous.