So within UK, a southern accent is an advantage, to be precise a Sussex accent, but in USA a UK northern accent is an advantage. I can move my accent and tried different ones to find good matches.
I worked in Alabama for a while and I found I got more success by adopting that accent, but I then worked in Texas and I found a UK northern accent best.
I semi-consciously control it, people hear me on the phone and notice I use different accents based on who I am talking to.
I know what you mean about a southern US accent being a disadvantage, it implies lower intellect, less educated, but it has the advantage, like in UK of a northern accent, of appearing more genuine, more honest. In USA my most enjoyable accent is New York.
In UK, for example if I go to north Wales, if I speak with other than a welsh accent I found I’m treated inferior, in northern Scotland I have to be also similarly perfectly highlander. But if say I want to be left alone in most places in England, speaking as a lowlands, e.g. Glaswegian, is a success.
I made the effort to learn French when I worked in Paris, then used it in Canada and they laughed at my Parisian accent. I’m not fluent, basically just ordering food+drinks. I learned German when working in Munich and tried it on another German who said I spoke with a Bavaria dialect. I cannot change my French or German accent, only my English.
In Montana I tried a few accents, they best understood my UK northern accent, so I stuck with that. UK northern is similar to Dutch accent, extended vowels, so I figured Montana was seeded by the Dutch. I never bothered to look up the history so I’m easily incorrect.
If you want a bit of fun…