Hey Gorgo, great to see some more prone foilers coming out!
I've been giving it a hack for about 6 months on the northern beaches, and your question is spot-on as I reckon I've wasted a couple of months trying to use a std volume 6' surfboard (~25 litres) badly retrofitted to carry a foil.
It made a huge difference once I committed to getting a custom high volume board (5'10" - 40+ litres) made by RMS with high strength fin boxes - I'll try to attach a photo

in otherwise unsurfable NE chop with my Iwa Gofoil.
I've made myself a shorter board - 5'0" using a Foilmount, and it's really responsive to weight input. The downside I've had is the total commitment to popping up, especially on a foam take off, lower volume/shorter boards demand. If I delay jumping to my feet, the dragging prone body weight behind the foil pulls massive 'back-stick' elevation, and you're back to the "bucking foil/death porpoise" experience everyone enjoys on their first waves.
cheers, Brian