Probably out of date now but I use Ultimate Spline Camera System, and then set up movement as follows for a character:
Probably don’t need the GET. The trick was the pitch being 0 on the Y axis for forward movement. Otherwise the player slows to a crawl if the camera becomes top down, for example if it’s tracking overhead and player is under it.
This works with splines and fixed cameras, so it’s basically like Resident Evil alt controls or early Final Fantasy 7 to 9 if you go for more overhead view. Seems to work just as well if your camera goes into side scrolling or third person shoulder cam too.
Anyway, that worked for me, maybe it will help someone.