EVERY entry to Thailand, including the first one or your 21st entry, is always at the discretion of the Immigration officer you are dealing with on the day you arrive.
Being able to enter Thailand on Visa Excemption is a privilege and not a right or ever garrenteed..
Stick to the rules as a genuine tourist and you should not have any problems..
Try to bend the rules or think you can and you will eventually have problems.
For Thailand you get a free visa exemption entry for up to 60 days on Australian passport.
Nothing to do before you arrive and just present your Australia passport to Thailand immigration and get stamped for entry.
For Vietnam you will definitely need a visa before you travel..
If you intend to go to Vietnam and then to Thailand and eventually return to Vietnam before flying home you will need a 90 day multiple entry visa for Vietnam..
This allows you to come and go from Vietnam as often as you like during that 90 day period.
There are jobs for English language teachers in Thailand but you really need some kind of teaching skills or qualifications for the best chance of a good job..
Don't expect western standard salaries either as these are not high paying jobs by western standards ..
If you are careful with your money you can survive on a teaching salary but that's about it..
Have a look at the DTV visa
"Destination Thailand Visa"
Depending on your financial situation and your longer term plans perhaps it's an option ?
But keep in mind you must be outside Thailand when you apply for a DTV and it can be done at a Thailand embassy in most counties.
Phnom Penh is a great city on the river and well worth a visit for a few days ..
Siem Reap and the Temples of Angkor are one of the biggest tourist destinations in SE Asia and attract millions of visitors every year..
And if you want something quite different head to the coast in Cambodia and check out Sihanoukville and Kampot and some lovely quiet beach areas along the coast..
You can even travel overland by bus from Sihanoukville back into Thailand via Ko Kong along the coast..
Interesting trip and onwards to Trat Thailand and even Koh Chang island Thailand and back to Pattaya or onwards to Bangkok.
I have been over most of Cambodia and it's a fascinating country and well worth a very good look.
The world is your oyster in Cambodia, Vietnam and Thailand and years of adventures to experience if you want to 😁