What’s the recommended platform to minimise cost for the long term but with occasional investing (quarterly basis) to buy US ETFs (US/Ireland domiciled)?

Hello there! You can choose interactive broker or Kristal to perform your quarterly investment. For kristal, there is no commission fees for ETF should your AUM is less than 50,000 USD. If your amount is more than 100,000 USD, interactive broker may seem a decent choice.