the Thai tax is on money remitted into Thailand. That means cash brought in, ATM withdrawals in Thailand, credit card purchases at stores in Thailand. Your salary into your Singaporean bank is not taxable unless you bring it in.
you're a tax resident after 180 days in a calendar year, whether or not your country has a DTA. What you pay or don't pay in Thai taxes is determined by the specifics of the DTA.
If you think you might do this in the future, start documenting your relationship now. Make sure you have plenty of pictures of the two of you together and document when you see each other, or communicate with each other.
With a retirement visa, you don't ever have to leave Thailand. Extending a DTV is a new process and we're still finding out how difficult or easy it will be. The monetary requirements are 800k vs 500k. If you extend a DTV in country, you'll still have to have 500k banked. Or, you'll have to leave the country.
For someone who can easily prove remote work and travels some, DTV might be a great choice. It's 5 years.
For someone living here who doesn't want to travel, the retirement route is well-worn and you pretty much know what to expect. You can easily buy a multiple re-entry permit if you travel.
And I am also concerned that the retirement financial requirements will change, but hopeful they will grandfather (an appropriate term here) those who are on it.