you actually had to cancel your ED-extension on Immigration with those paper documents of your university, before you left Thailand. So now the game is open, you will probably succeed - because the embassy in Cambodia can't see your multi entry ED stay permit - but Immigration will be seeing it on their central computer, and it is an unfinished process, so perhaps it's Immigration that will cause a problem
not really missing anything, because some Immigrations actually refuse the last 30-days extension and send you home with a "you got 7 days to leave the kingdom" stamp. Those "almost 9 months" are not guaranteed (because nobody flies to Thailand on the exact date their METV gets issued)
And, even after all these discussions in the group, you are still in the wrong!
You said you paid 300.- AUD visa fee
This would mean that you MOST OBVIOUSLY were issued a 6-months multi entry Tourist Visa. Check the photo below. It lists the visa fees of the Thai General Consulate Sidney
The visa validity is 6 months from the date it is issued. Each entry stamps you in for 60 days. You can extend every 60 days stay permit with 30 more days on Immigration for 1900.- THB.
If you exit and re-enter Thailand short of the expiry date of the visa, you will get stamped in for 60 days for a last time. By this you can get almost 8 months stay in Thailand out of this visa
So would you please have a look at the pdf. visa document you printed, and tell us how long the visa validity is? It is the date printed after the text “visa valid until”
you got a Non-Imm-OA visa, not a Non-Imm-O. All Non-Imm-O visa categories are pdf. printouts, no more stickers. If you would apply in Australia for a new Non-Imm-O/A visa, it too won't be a sticker any more
ah, my bad! You are a male foreigner who wants to get married to a Thai female. I confusedly thought you were a foreign female. So ignore my remarks about not needing the financial proof. I already changed the text accordingly. Once married, you will need the 400.000 THB in your Thai bank account for the "married visa" and the subsequent 1-year Extension of Stay