You should definitely not choose the 85 for "learning" purposes.
You need (quite some) more wind when learning anyways, so not much gain in this period now you can go in both directions, as a smaller board is usually easy when windy.
I am 78 kg and would probably choose 85 instead of 75, as I found so many advantages and no disadvantages when board is light (and nose is light), and I only want one board.
A few of the top dogs has a 60 or 70 litre board (guys weighing 80-85 kg), but they have at least two boards, and never use the small ones when wind on the light side.
Has happened often they come ashore (after a long time, sitting on a small board trying to get ashore fast) and pick a bigger board.
No problem if you have two boards, but unless you KNOW you wont have lulls or softer wind, I really dislike too small a board.
Amazed to read in the other thread that some have 30 or 40 litre boards (only?), as this is not what works for anyone here, not even the top freestylers...
But as said, area is more important than volume IMO, so not too short and thick seems to work a lot better (for me)
Cant tell you what to do, only that you should NOT choose the 85 to speed learning up.
You should choose it, or the 75, if you think it is your best "one board for all" when you are experienced
Some of us "average to okay good foilers" prefer 75 l, others 80 l, and then some 85 l, if average weight, but bigger than that, no way, and smaller rarely or not used.
The outline and length and weight is a lot more important though, than the volume.