With plain dynema, pretty much any knot will pull through if sufficiently loaded. This, I think, will be the issue, rather than the knot weakening the line.
To overcome this, I would use a line with a dynema core and polyester cover or even just a plain polyester braid line (@5mm you should get 500kg plus). Knots should hold in these ok.If you worried about strength just go up a size - a larger diameter will be kinder to the webbing loops on your harness.
Personally for joining the ends I’d go for a double fisherman’s (
https://www.animatedknots.com/double-fi ... -bend-knot). I guess the overhand knot that you have shown for the QR is ok but not very elegant. It might move around a bit and wear the line?
Here is a possible alternative (using double fisherman’s) [ignore the loop in the end of the green line - it just a bit I had lying around]
Or even
But not sure how the shackle will take the sideways loading? Probably of for foiling.
Are you dead set on not using a spreader bar? Lots of folk (myself included) cut off the hook and connect to a rope loop attached to the center of the spreader. Similar, in some ways, to what you have??