Proper resignation date based on relocation date for spouse
I'll need to deal with the inevitable peer negativity and rumor mill that'll spring up.


You must work in a really bitchy department if you get peer negativity and rumors just for wanting to pursue an alternative career path.  In a well-functioning department, we wish departing staff all the best, and thank them for their service.  In any case,  whether or not you work in a department that is actually that bad, I can't imagine it will be any better if you leave more suddenly, with less notice of your departure.  The longer you leave it, the less time people will have to fill your duties, and the more inconvenience to them.  I'm sure your department head and close colleagues would appreciate as much notice as possible of your departure, and if you want this to remain confidential until you have some "official notice" of leaving, you can always ask them to delay telling the rest of your department.

Academics leave university departments all the time, and in this kind of circumstance, I would not expect any negativity to be directed towards the person leaving, particularly if they have given a substantial amount of notice.  If your notice of departure genuinely causes peer negativity and adverse rumours, that would reflect badly on the department, and it would just confirm what a good decision it is for you to leave.  If you are sure about leaving, my advice would be to give as much notice as possible --- i.e., let your department head know ASAP.