The US taxman doesn’t care about UK tax-free wrappers, so unfortunately you pay 15% on US shares dividends, but I believe that’s where it ends, i.e. no CGT.
Yes, the tax on dividends is 30% but a treaty between the US and UK reduced it to 15%. You have to fill out the US tax form W8-BEN to be verified that you qualify for that reduction.
In an ISA, the 15% american withholding tax is deducted from your dividend payout before it’s paid and there’s no avoiding it. That’s as low as you can get in an ISA.
However, in a SIPP it can be 0% percent if your broker is a Qualified Intermediary with the IRS. Old school brokers like Hargreaves Lansdown and AJ Bell are 0% but Freetrade (or should it be Gaudi) is 15%.