First: The re-entry permit lets you come back using the same stay as you have now, you will get the same Admitted to stay date stamped on entry, it seems no option for you at the moment as you leave for 3 months.
You came to Thailand using a Single Entry Non Immigrant O visa based on marriage and got 90 days of stay.
You are still in this 90 day period.
If you leave before the Admitted until date, this visa and stay will be cancelled and you have to apply for a new visa when you decide to come back to Thailand.
OR
you can apply for the 1 year Extension of Stay based on marriage, when stamped in the passport you apply for a re-entry permit and you can visit Australia and come back using the re-entry permit.
If you apply in the Status you get 90 days on entry and have time to arrange the copies for the Extension of Stay.
If you enter with the Visa Exempt Entry, you have 15 days to apply for the conversion to a Non Immigrant O visa, and 60 days later you have to apply for the Extension of Stay.
You can NOT apply for a tourist visa inside Thailand.
If your Non Immigrant B visa or Extension of Stay based on employment stops due to no more working, you have to leave the country or meet requirements of another Extension of Stay. If you apply and can not meet any requirements you get application not approved and 7 days to leave the country.
You can not change the reason from an issued visa.
You might cancel or let it run out, the current Extension of Stay based on education and apply for a new Extension of Stay based on being over 50 years of age.
But this all depend to the visa you used to enter Thailand, easiest option is to visit your local Immigration Office with your passport and ask if it is possible and what the requirements are.