My take on this, windfoiling/wingfoiling/kitefoiling, SUPfoiled once but I havent surffoiled.
Easiest to most difficult in order:
1. Wingfoil.
2. Windfoil.
3. SUPfoil.
4. Surffoil
5. Kitefoil.
Based on the fact that kitefoil is by far the most lively board and kite power is tricky to apply correctly.
Wingfoil is very intuitive, board is big and slow, and you can just progress as slow as you like.
Windfoil a bit more complicated as the mast foot pressure adds a new aspect. For an experienced windsurfer of course.
SUPfoil might be easier to learn than windfoil, or about the same, I dont know?
Surffoil requires steeper waves and I think it can be quite challenging controlling speed and height on a steep wave.
Just my take on this, and you have to try to look away from the order you learned things yourself, as of course the first foil you do is difficult.
My order were: Kitefoil, Windfoil, Wingfoil, SÚPfoil, no Surffoil.
Peter