It's entirely optional WHERE you place your
myadmin dir. Personally, I would NOT place it inside your main public
files dir ... You could for example place it on the same level as your main files dir, and then you would use
'root' => '../files'.
_filesconfig.php file should only be inside the admin dir, because it would allow the Files app that is there, to NOT create it's own cache/config dir.
It's a bit hard to diagnose from here without seeing what's going on ...