When is the earliest I can apply for a multi-trip 60-day visa for Thailand if traveling in November?

On the 6 month METV, it is advantageous to apply for it as close to when you're traveling as you can

that's because the 6 month validity of the visa starts counting down the day the visa is issued. So if you get it too early you burn the validity of the visa without being in thailand.

I'd say apply when you're about 30 days from your trip. Take your time on the eVisa application, get everything filled out correctly, check the data and the uploads you submit and it should go smoothly