So long as the tide has a coefficient of at least 80 then it should be high enough to go to Mont St. Michel. Use this link to determine coefficients for a seven-day period: http://tide.frbateaux.net/48
High tide at Cancale is more or less the same as that at Mont St. Michel (15mins before HW at St. Helier pier heads).
The jetty in the photo dries at about 8.5m. It's better to go on a small tide and creep in than go on a big spring and have plenty of water because if you stay the day the flood in the evening is that much weaker. You tend to get pushed about a bit alongside the jetty until you float. I might try Martini II's suggestion of an offset bow anchor next time.
Another thing to remember is that the seabead near Mont St. Michel has changed quite a bit over the winter as has the approach transit to the Mount itself. I have waypoints for this new route on my chartplotter. I plan to go back to Avranches on the big autumn tides. It'd be great if some other boats came as well.