I do think that Barkley has real problems with confidence.
This has culminated in him losing his identity as a player; as was rightly mentioned, he rarely drives from midfield beating players with his power and pace anymore.
He has been effected by having two contrasting managers, and this has messed with his head too.
I guess you have to question what we want from him.
I want him to drive, beat players, shoot more, look for forward passes, and press.
The trouble is, I'm not sure what Koeman instructs him to do (other than the preshing and forward pashes... sorry ☺️)
I dn't think Koeman is asking for anything different than any other top flight manager asks from his supposed flair players. If he can't grasp Koeman's instructions he's even less chance of Pochettino's.
The issue as you say is Ross himself. Is he bright enough to grasp the nettle on this and take his game onto the next level. Plenty of players change managers, it shouldn't really affect your game any more than a few transitional months.