When can I apply for a new visa extension in Thailand if I have to leave the country before the current one expires?

Bcause you didn't get the funds in the account until Sept 14 the earliest you can apply would be the 14 of Nov 🙁 <- that is a problem of your own making sadly because in Chiang Mai you can apply when you have 45 days or less left on your current stamp so had you had the funds seasoned the requisite time you could have applied any time after Oct 10th 😕

You're talking about the 'under consideration' stamp they give you (which is actually the first month of your new yearly extension). You can leave during that time but you'd need a re-entry permit to get back in and you'd have to be back BY the date the under consideration stamp runs out.

If you're going to apply before you leave, TALK to them, let them know you have to leave the country ask if you can get a longer under consideration stamp (they have been known to be accommodating in some cases).

If you can't get back before the u/c stamp runs out you lose the whole thing and start all over from scratch.

Come in visa exempt, apply for the 90 day Non-O for 2000baht at the immigration office, wait the 20 day review period, then wait until you have 45 days or less left on the new Non-O and 90 day stamp to apply for the year extension for 1900baht.