While 70l is less than 25 kilos your weight, I would absolutely go for it.
The riding experience will just be so much satisfying on a smaller board.
Just use a larger foil and wing sizes during the first days, until you get used.
If your home spot is difficult, with fast currents and shifty winds, I would go with a low volume mid-length. The mid length are easier to launch in tricky conditions.
Regarding the brands, there are experts on this forum who will provide better advice but if you are riding Armstrong foils, I'd personally stick to the same band