I haven't read all the answers but I've already heard your question a hundred times. Basically, can I skip the big expensive boards since the dream board I'll be riding in 3 months is going to cost me a lot of money to get to.
I had all the boards. I don't regret it. And the used market is pretty hot with so many new wingers, so I'd suggest going the long road (recommended board sizes).
You actually get a lot better as well learning how to milk a board that you begin to consider "too big"...
And, the board has a lot less impact on the riding than the foil and and then the wing. The board comes in third.
An added bonus is that most everyone I know who loves winging also loves sup foiling so the board doubles as a sup foil board.
If you are still convinced you want to go smaller it is certainly doable albeit painful.