How would any earnings prior to taking up residence in Thailand be treated? If I took up residence in 2025, would I only start to be taxed on capital gains earned in 2025 onwards?

It depends on the type of structure you hold. Cash in the bank from previous tax years when you were a non-Thai tax resident is not assessable income. If it is an investment fund then you would be liable for the capital gains on the investment fund, not from the date you moved to Thailand. In most cases there is no relief for when you were a non-tax resident. Please seek a 1-1 consultation to discuss specifics and before taking action.