Clogging could be a ...

Clogging could be a problem,unlike for vertical flow constructed wetlands,though for simple greywaters from showers,horizontal flow SSFCW should theoretically suffice,in combo with a septic tank,as u indicated.

But with 65m2 area,as per your calculations,it is fairly large,making it more prone to frequent clogging.

Hair should mineralize without problems in vertical flow systems,I feel.

Moreover,expert maintenance may be required,unless u use proven systems,as may be possible,to have,readily.

Moreover suspended solids are not well removed in septic tank,which is more suited to higher denitrification requirements,which is not so much the case with grey water alone,with the organics.

If organic load is higher multi stage,higher surface area vertical flow constructed wetlands can be considered with subfilters.