Good day
In general, there are of course many American trading platforms/brokers that are cheaper than Swiss platforms/ banks.
But if you can no longer trade in the USA, and want to trade regularly, there is no need for a comparison between Swiss and US providers. So the recommendation is important that you choose a broker that is as cheap as possible (by Swiss standards). You can find the Swiss broker comparison on moneyland.ch: https://www.moneyland.ch/en/online-trading-comparison.
However, if you can still hold the US shares with your US broker (and sell them at some point), this is probably the cheaper option (unless you have to pay higher custody fees there, but this is often free in the US). This is because you would have to transfer the shares to a Swiss account (usually in CHF), in which case exchange rate costs would be incurred.
Or you can avoid these by opening a USD account with a Swiss broker. But even then, Swiss brokers almost always charge custody fees and/or inactivity fees if you do not trade. Cornèrtrader has no custody fees, but inactivity fees - Cornèrtrader could be an option if you buy a share every now and then.
Best regards