https://tp.consular.go.th/homeApplicants can use their approved Thailand Pass QR Code to enter Thailand on different date of arrival if the new date of arrival is within 7 days before or after the original date of arrival as indicated on the QR Code
Looks like since you have to attach your vaccination certificate, only thing you should care besides insurance, - your certificate that you will have at the day of your application for ThaiPass, should be still valid in October.
And by the way: at time of applying for ThaiPass you have to have your flight details already: day and flight number