@georgehay IKBR is a good broker.
In order to increase the breadth of your investments you may think to trade options. You could start easy when applying the wheel strategy to the stocks you own and want to own, especially if you have at least 100 of them. This is a relatively easy way with relatively low risk to make significant additional profit, something like 3-10% p.a. in addition to your stock, this means you get extra if the price rises and you dampen the loss if the price decreases. You can also apply this to ETFs that you own and want to own and that can be traded on US stock exchanges, for instance the SPY (an ETF that mirrors the S&P500).
In case you speak German, I recommend the books from Eric Ludwig: "Optionen unschlagbar handeln" and the one dealing with hedging via options. These books are the best I have seen on the market, incl. the ones in English language. Also his Youtube channel is very helpful.
If you do, inform yourself regarding the risks, you need a certain degree of discipline here.
In my case - I was emotionally not able to deal with the ups and downs of the stock market, however, after switching to option trading I am way more calm and satisfied and also believe that I have found a more predictable and profitable investment vehicle than stocks and ETFs.
Tax - in Switzerland you do not pay taxes on profit derived from stocks that you invest privately, only on dividends (and for instance on stock options that some people receive from their companies and that they ecxercise).