I think you should consider the 93 liter Futura as your next board size down. At 70 kg & 20 knots the 111 would still be bouncy in choppy waters. You should also consider sail range. The 111 would probably be 5.5-8.0 for you which would leave an lot of sail overlap with the two boards.
I weigh around 80 kg and use the 93 & 133 Futuras. The 93 sail range for me is 4.5-6.6 & the 133 is 6.6-8.5 so I only have one sail overlap.