I think the 36 cm Lessacher Duo will do all that Wolfgang suggests.
I think James may be correct that your stock 50 cm fin may not be large
enough for your 9.8 rig.
The 58 cm should be OK in lighter winds, but you my get some tailwalking if you
use a 58 cm when fully powered to overpowered on the 9.8.
What 40 cm weed fin do you have.
Some 40cm weed fins will work with big sails, some don't.
Hope this helps,