Can an expat declare their income and pay taxes in Thailand with a DTV visa?

Visa and tax are unrelated to each other, when you are 180 days or more in total in a calendar year in Thailand, you will be considered tax resident, and might have to pay taxes depending on your situation, but not on your visa.

I can't get a Thai bank account on DTV. It is not possible. So I suspect it will be hard to assess tax liability without this. I'll just have to live off my wife

You automatically become Thai tax resident after spending, consecutively or not, 180 days inside the Kingdom in a given calendar year. Domestic income is taxed from day 1. Foreign sourced income is liable for tax once you become tax resident. Only “assessable” income is taxed. What is assessable income depends on the source of the funds, your tax residency status and the wording of any double taxation treaty in place. A remittance is classed as any interaction with the Thai banking system so this includes ATM withdrawals, money transfers etc. Use of credit is not a taxable event. If you are Thai tax resident and remit assessable income during a tax year above minimum thresholds you are required to file personal income tax by end of March in the following year, there is a small extension if filed online. The fact that you hold a DTV is irrelevant.

You're a fiscal resident when staying 180 days and more per calendar year whatever the visa or exemption you're holding.

The rule is that if you spend more than 180 days in a calendar year in Thailand, you are liable to pay income tax in Thailand.

What visa you're on, what you do for work, how many sheep you own or how often you fly in and out has no effect on this rule.

Go to your local tax office and get a tax identification number so you can pay your taxes in Thailand.

The "others are saying" part of your post is just for people who have just come to Thailand and don't want to become tax residents before they've been here for 180 days first. They still have to pay taxes here after that.

Yes. If you spend 180 days or more inside Thailand in any calendar year then you are a Thai tax resident in that year. If you are due to pay tax you must do so by the end of March the following year. How much tax you have to pay, if any, will depend on how much income you bring into Thailand that year, whether you can claim for any tax already paid in another country and so on