You can never do your first 90-day report online. So doing it online won't work with a DTV, unless you're extending and staying longer than 180 days.
And the 90-day report doesn't matter for you, if you're flying out soon. It is only important if you have anything to do at an immigration office. The immigration at the airports don't care about the 90-day report.
Anonymous participant if you APPLY for a visa and say you're going to fly more three months in the future, tour application will be denied. This is to prevent people from making the queues at the embassies even longer bu applying too early.
But nowhere is there a rule saying you have to use your visa within three months.
That is the whole purpose of the visa. To be able to stay or come and go as you want, for 5 years.
But you might want to fly out and in instead of doing extensions, as it's A LOT easier to just leave and come back than to deal with immigration offices and their weird requirements for extensions.
you go to your local tax office and ask what you need for a TIN. This is Thailand, there is no general rule for every location, nothing is consistent in this country. So you'll have to see what your particular tax office needs.
Basic documents they will ask for are a bank statement from your thai bank, a work contract showing income and an address in Thailand.
Now, because this is Thailand, there is no way for them to enforce this rule and make you pay taxes here, yet, since they don't check how much money you bring into the country unless you report it yourself. They do however have the right and international agreements to check your accounts in other countries. The chance of this happening is probably around 1% right now though.