unless you can clearly show the transfer to a new account maintains the original funds then yes you will have a problem. The Netherlands and Thailand have a dual tax agreement (DTA) but it doesn't give you an exemption from tax in Thailand. If you bring income into Thailand then that becomes assessable income for Thai tax purposes which means you need to file a Thai tax return. You can then claim a pro rata credit against Thai tax for the tax already paid in the Netherlands on the income you bring into Thailand
Nothing to worry about. Your evisa will show 5 years just as a sticker does. You get a 180 day stamp every time you enter during the validity of the visa
a stock portfolio won't work only money in a bank account. I don't know how Dutch tax assessments work but in the UK you don't enter savings in a bank account, only the interest received on the savings. Save a copy of your bank account statement covering the end of 2023, then you need to show that the money you bring in to Thailand comes from that bank account which you can again show from statements. You can't let the money in the account drop by using it elsewhere and then topping up the account again.
you'll have no problem with getting an extension on your current entry and will be fine with a border run and further extension as long as you have no recent history of staying in Thailand
what do you mean by savings of any kind? If you had savings in a bank account as of
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/2023 and can show proof of that, and only bring THAT money into Thailand then you are not liable for Thai tax. Only income is subject to tax. Bear in mind you might need to prove this to the Thai Revenue Department should they wish to audit you.