Can you also consider alternative solutions which provide better isolation from mains voltages such as (1) a low voltage fan or, if it must be at mains voltage, (2) a solution with an optically isolated triac/diac pair such as described here: Dimming effect by zero crossing detector and a triac
You can also get ready made modules: https://robotdyn.com/ac-light-dimmer-module-1-channel-3-3v-5v-logic-ac-50-60hz-220v-110v.html
Even if you get your current solution working satisfactorily, absolutely everything has to be insulated to prevent the user coming into contact with any otherwise exposed metal parts.