Holding any elected position should come with a lifetime ban against holding stock in any publicly traded company, full stop. Congressman, president, sheriff, county commissioner, I don't care. Either you're a public servant or you aren't. My position in the government wasn't even an elected one and I still believe the oath I took is fundamentally incompatible with holding stock.
Placing restrictions on what investments a person holding public office can hold is a very naive solution. Instead, acting on material nonpublic information is what should be against the law.