Don't rush into a DTV if you arent sure what your plan is within Thailand. Your signing up for 5 years of no legal work within Thailand, other than international remote work, and its not an easy thing to change from 1 active visa type to another, Its not like existing visa's where you can simply do it at extension time changing the reason, or leaving the country without a re-entry permit to cancel it.
its two different things, the audit process, filing and fine rules have been in place for ever and remain unchanged. In case of an audit, which can be done on any tax resident, they reserve the right to go back to check 5 years finances, but if no tax returns have been filed they can look back over 10 years. But this has always been the case not new news. Do they or would they is another matter.
the tax liability between global and remitence based filing shouldn't be that different, just more ink if your from a country with DTA as you have to record more and credit more.
It can be an advantage, because you should be able to get a non-tax code in your home country and actually only pay tax in Thailand for some earnings. You generally can't get one for remitence based countries, as too much money can remain untaxed if left in source country.
uk state pensions have always been assessable income in Thailand, and always subject to tax as no tax was paid in uk. Previously if you saved it for a year before sending it to Thailand it was tax free here, but not anymore. If you dont file and pay thats upto you, but you will still owe it.
historically this is probably quite low, but for 1 of 3 reasons, ignorance of tax laws, only remitting non-assessable income or evaluation of risk of being audited. The change this year is on the non-assessable remittence, which has made some previously non assessable become assessable. This doesn't mean you will pay tax on it, but does mean you should do the return and use a DTA aggreement to negate the tax along with Thai tax allowances.