I used to translate I'm back as "Je suis retourné" but eventually I changed that to "Je suis de retour." Am I right nowadays, is that the most idiomatic way of expressing that?
Yes, "Je suis de retour" is a correct translation. You could also use "Me revoilà" which is, I think, a bit less formal. In a very casual context (with friends or on an online chat for example), you can also simply use "Re" which is a slang for "Retour", or "Re-bonjour". I don't know the exact origin of this last one but it's definitely used to say "I'm back".
If you are talking about a move from or to a location, it can be translate like that "I'm back home" means "Je suis rentré/revenu/de retour à la maison"
If you are talking about presence (for example on a chat), "I'm back" means "Je suis revenu/de retour"
You can say "Je suis retourné" when there is a location or an action just after. "Je suis retourné en France" means "I'm back in France" and "Je suis retourné au travail" means "I'm back to work"
Take care of your translation because "Je suis retourné" has a strictly different signification. I couldn't find how to say this in English. It means that I turn my opinions about my country. The best example I have comes from Homeland. Broody a été retourné contre les États-Unis.