Granted I don't run modern Android but being on a custom ROM vs OEM stock has nothing to do with whether or not one should root. Being on a custom ROM doesn't mean you have more control over your device. Where possible I have always been rooted (since Jellybean). Only on rare occasions (device cannot be unlocked) am I not on a custom ROM. I need / want full control over my devices.
I am, however, one of those persons who does not tell someone they should root nor am I one to tell someone they don't need root. That's up to the individual and their needs. The old adage that if one has to ask whether or not one should root then the answer is probably no.
I only haxe one phone with Thunderbird. Checked but I see no way to add folders. Since TB is based on K-9 Mail I assume the latter can't do it either but will check. FairEmail, however, does allow for creation of folders.