Can I extend my TR Covid visa at Jomtien Immigration Office and do I need to update my 90-day report at the same time?

Both "Jomtien" and "Racha" offices are considered "Chonburi Immigration Offices".

>I was wondering will I be able to use the jomtien immigration office for another extension before Sept 27th end date

You should be able to, since they are the "same" immigration office.

> I did my 90 day report at si racha july 6th,

You need to file a 90 day report every 90 days at the immigration office. It needs to be submitted no later then October 4. You can submit the report 15 days before the due date, to 7 days after the due date in person.