There’s no stamp duty on US assets. The W8-BEN reduces the amount of US withholding tax from 30% to 15%. You only need to complete it once, though if your US citizenship status changes in the future you will need to inform every broker through which you have completed the form.
You still need to pay UK dividend and capital gains taxes on US stocks if you go over the allowances, which are currently £2,000 and £12,300 respectively. You can avoid these taxes by holding the shares in an ISA.
You usually don’t need to report your capital gains and dividends if they are below the allowances but there is an exception if the total amount of share sales is more than four times the allowance even if your total gains are within the allowance.
@Conladx gave a good example of how this works here:
The government guidelines on reporting and paying capital gains and dividend taxes are here: