In Chiang Mai mine was indeed reser when I got a new Visa extension. I went to do my 90 day report and the officer told me I was too early as they had a record of where I was staying when I had applied for my Non-o extension. It seems to differ slightly but as a rule I would take it as 90 days from the day I last entered Thailand.
You don't say your nationality but basically each of you have to submit your own individual applications to the embassy of your destination country from the country where you reside. There are exceptions but to travel to the UK, Europe or USA etc that is generally the case. First step is to decide where you want to go.
Just an idea but can Admin pin the actual requirements and then put a link in the comment each time this question is asked to stop everyone posting thoughts and what they heard in a pub etc...
I had a similar situation in Chiang Mai last year and was told it's 90 days from when you were last at immigration ie for you use 6th Dec date as its 90 days from your last notification at immigration. This may cause flak as everyone else seems to have said 90 days since you arrived ....... helmet on, incoming 👀
I was more wondering why the guy thought you would only get 30 days on re-entry. When my son was made redundant due to covid the company actually didn't close his work permit to allow him time to find alternative work which worked out well as it picked up again and they just restarted him as though nothing happened. Hopefully you get sorted again soon.