It would be awesome if there was a central website with conferences listed, but I’ve not found one. The two big conferences that are hygiene focused are the ADHA conference (which you found) and RDH Under One Roof. RDH UOR is put on by Pennwell (they put out RDH Magazine). It is held just after the ADHA conference in the summer. Beyond that each state tends to have their own dental conference each year (not hygiene focused). For instance, Midwinter is a big one in Chicago.