I haven't sailed an AI, but I assume much of your downwind issue is a "taco" shape of the boomless sail. For one thing it is common advice to avoid pointing straight downwind but do a slight zig zag. This sometimes increases your progress downwind (not just your forward speed). This will also allow a slightly better sail shape, especially if you "sail by the lee". That means pulling your sail in enough to reverse the flow from back to front. The tension also shapes your taco sail flatter.
The 2 above approaches didn't really speed up my Hobie inflatable kayak with boomless sail, but should work with more efficient rigs. You can research these approaches online. I found sailing by the lee very entertaining and challenging with gusty powerup and all, but can't remember details. There is also a huge thread here years old named something like "what a difference a boom makes".
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