my fin was a 30 cm Tribord freeride (from Decathlon).
Your reply seems to confirm to me that the first problem was given by the sail.
My personal opinion is that the 120 lt. board could'nt be considered the main problem in a starting non-planing phase (you can keep it easily down). . on the contrary it should help with the shore break . .
Do you agree ?
Thank you, Enrico