Big Toe.. going numb

Have you changed your shoes? If so cleat may not be setup as well as it was previously. It may also be that the shoe is a narrower fit at the toe end effectively squashing your toes.

Have you changed anything else about your bike setup? Raised or lowered the saddle, etc as this may have changed the pedalling dynamic ie you may twist the foot more within the range of float that the cleat allows. Did you change the cranks for longer or shorter ones in which case your fore/aft saddle position would need adjusting along with the saddle height in response to the change in crank length.

Are you wearing tighter fitting socks? It may be simply that your socks are getting pulled tight inside your shoe and this is efffectively strangling your foot.

How long were you riding with the same setup prior to the problem arising? If its only a short time it may be that it is only starting to now manifest itself more clearly or your time on the bike on a single ride may have increased to the point where the numbness starts and previously the rides were shorter and thus the toe never got a chance to get to the point of numbness - it just worked up towards that point but never actually got there.

Have you started adding more sprint or low cadence work to your routine ie are you putting more force through the pedals than you would have previously? If so a firmer sole may help (carbon preferably).

FWIW my overshoes make my shoes just that bit too tight with thicker socks on. It's ok with thin socks and overshoes, or thick socks and no overshoes.