sound is no problem with your footstraps positions on the iSonic 111. if you feel „like the rig is falling backwards a bit” the first solution would be pull your mastfoot more back and try every 0,5 cm until the bottom of the sail will be (more or less) parallel with the deck in reaching and your bad feeling is disappearing.
if after that still did not change your feeling, your fin should be too small and/or too hard, but this problem with your 44cm fin in the iS 111 is impossible.
so the last chance from my side is you over pull the outhaul of your sails and this is why the rig move to much back. the smallest gap between the leeward side of your boom and your sail (in planning) should be between 0-3cm. if the gap is bigger the pulling point of the sail start to move forward and the rig start to move backward – to keep the balance.
let me know the result of your check,