Unlikely you'll need to file a tax return in Thailand if you have already paid tax on your "remittances" as differences would be so minimal it would easily be covered by the deductions. If the Thai tax office enforced this detail, they would be inundated with forms which ultimately would cost them several thousand baht to process with nil return
No there isn't, it's based on a number of factors, all of which are ambiguous. There is a proposal to tighten the criteria which at its base will be Australian Citizenship and 45 days in the country in every two year period. It's extremely easy to maintain residency in Australia.
Agreed it depends on individual circumstances. I'm retired but I travel overseas frequently (I haven't done a 90-day report since covid), so the DTV is better for me on two fronts, it's cheaper plus no visits to immigration required. I have an investment property portfolio in Australia, so I've got a registered business (I'm the only employee) which administers the properties. This "job" can be done remotely so I qualify as a "remote worker" under the DTV guidelines. You are correct that Australian pensions are not taxable in Thailand, provided the recipient retains Australian Tax Residency which I easily do, so taxation in Thailand is not an issue for me
I'd pay for the visa in Australia. However, this thread is about paying tax on remittances to Thailand. If there's no trace of any remittances there's no tax