To achieve good tube & transom rigidity. Was your sib well top inflated ?
Transom plate seems not well centered, probably just a photo issue ?
Did you visually check tail height passing throught water flow ?
Try removing doel fin from leg, that horse is very punchy, doel fins are used in most cases to help plane underpowered engines with badly ballanced sibs/ribs. I have finally found the proper place to store mine forever, the waiste basket.
As to understand Tohatsu water issues, check pic. Where are you right now. Is your 18 L same as in pic ?