I've got the same board, but with a Moses 1100 foil setup. Also just taking first flights but found similar to you hard to keep nose down. I moved mast all the way back in the track which really helped. Played with shimming a bit, but felt really loose in yaw.

I've found I just have to concentrate ...

FWIW: had a great learning session (my ~5th session) yesterday. Key points that seemed to be helping get first few flights:
- Bend knees rather than trying to stand straight up, or leaning out like my windsurfing muscle memory trying to do
- Mast location depends on boards and foil. 5'8" Quatro ...

I'm 78kg and got the 115L Quattro Wingdrifter from DirtyMermaid in Ucluelet. Really liking it, and quite stable with a giant Moses 1100 foil underneath. Tried SUP paddling, and the 5'8" length definitely makes going straight hard. Haven't tried catching a wave yet though; was trying for flat water ...

I'm liking my 6m Ozone Wasp V1 so far. Plan to get a 4m if wife/life/kids allow me to get out much on higher wind days.
Bigger wings sacrificing aspect ratio to fit the wingspan in, so I'd expect diminishing returns with size.

Finally got my board & foil, from two separate shops who actually had stock, but neither came with T-nuts to attach said foil to board!
Anyone know anywhere local to get those?

Thanks for all the hints! I picked up a 6m WASP over the summer to play with on the SUP.

Trying to source board and foil (never foiled before, WS non-foil in the past). Anyone on VI have a 'learner' foil set they're done with? Looking I think for 2000cm^2 to get going, and try some flat water ...

Has a thru thickness solid section underfoot.
If stiff enough I really Iike portability piece and once on a foil the board shape much less important.

https://youtu.be/GkhS2fDyuQs