How would you amend the Constitution to avoid this? Meglomaniacs get elected to Congress too, so making it a requirement to be a sitting representative to be elected Speaker wouldn't help.

To be honest, I'm not sure what changes we could make to our Constitution, laws and customs to prevent Trump or anyone like him from destroying our democracy. Democracy depends on everyone following the rules. This has never been tested until Trump came along. And he showed how easy it is to just ignore the Constitution, our laws and the custome surrounding the presidency for both personal power and enrichment.