I’m, a Dutch national, can I reduce Thai tax by planning before becoming a tax resident?

Yes. If you sell and repurchase assets in a year you do not spend 180 days in Thailand, the ‘cost base’ potentially resets to the new purchase price. This can reduce future Thai tax when you eventually remit gains, but may have overseas tax consequences which need to be assessed. Pre-move planning is often the most effective way to avoid unnecessary double taxation.