AnyBoard said..
The idea of a Midlength is to ride as minimal sized board that can get up easy. If you need something that much above your weight all good for you but your 120l is more a Midlength for someone 6'5 and 110kgs. At your size it's even too big for a sup foil board. It's boat size but everybody is different I guess.
I wonder how corky it would be out of that lake and in some ocean conditions.
That's the beauty of the mid length concept - the versatility. That is, if the shape is scaled up and down sensibly, not making it too narrow if the volume increases. If it's too narrow it just gets less user friendly.
If a 100+ kg guy (a beginner/intermediate wing foiler) is with a 8 m2 wing and 2400 cm2 foil and 120 L board, then I am with a 60 L, a 3.0 wing, and with a rather sensibly sized foil. Same conditions, same board model, both having a blast.

AnyBoard said..
I wonder how corky it would be out of that lake and in some ocean conditions.
It's not in the video, but the only day I have been in the waves with the 120 L was on that day, with a 2.9 BRM parawing and with a 700 cm2 foil. Easy, stable, super simple to get going, not corky at all. Right after that I used a 60 L board and 2.5 m2 wing and 680 cm2 foil in the very same conditions, here in this clip. Maybe your conditions are so much different?