Snap shackles on dock lines just don't make sense.... if they did you'd see them elsewhere. The fixation point needs to be firm and immovable, so the flexible dock line can absorb all the stress. I don't like the cleats you pictured either, but even a bolted on bow shackle or pad eye, that was fixed in place, would allow the large diameter dock line to pass smoothly through and be tied off with a traditional knot. A snap shackle at the end of a rope is going to rest in an unnatural position, and the constant flexing that dock lines exhibit 24/7 will be noisy and abrasive. A genoa lead block shackled to the toe rail has all its load going vertical... and the load, in general is consistent when the sheet is active, and non existent when the sheet is passive.
Anyway, good luck... hope it works well.