Do I need to show 5,000 THB in a single bank account for a DTV application? And is there a tax obligation after extending a DTV?

2) After 180 days in Thailand, consecutive or not, in a calendar year you automatically become Thai tax resident, that’s all. Whether you have to get a TIN and file Thai taxes then depends on whether you have remitted “assessable income” above minimum thresholds. What constitutes assessable income depends on the source of the remittance and any double taxation agreement that is in place. So the answer is you may, or you may not, have a Thai tax liability depending on your personal financial situation.

It can be from different accounts.

Yes and yes. It doesn’t have to be a Thai account. Perhaps research on here. Plus you might be flagged.