Sorry for the delay! I've been working this week on the other coast of the USA (3 hr time difference).
OK, if you are going to only have one board for 8 knots-22 knots, a wide 115-125 liter board would
seem to fit the conditions. How wide.... => 85 cm wide.
At 75 Kg. you can uphaul a 115-125 liter board fairly easily.
6.0m2/7.0-7.5 m2/ 8.5-9.0 m2 would be my choices.
May not get planing real easy in 8 knots of wind, even with a 9.0 on an 85 cm wide board, but
@9-10 knots you should plane fairly easily.
At 22 knots, a well tuned 6.0m2 should still be manageable.
Hope this helps,