Im thinking 10 would be the best place for him currently.Ideally you play him as the 10. Gana of course infront of back 4 intercepting counters and then Garner as the continuity 8 type.
However Doucoure has been one of the successes of the Dyche reign so far and while we all know his limitations on the ball he gets on the end of stuff and is a reasonable scoring threat (certainly more than Onana).
So Onana is being asked to play the role that should be given to Garner.
If Moyes was here I bet he'd play him as CF and morph him into Fellaini. Couldn't be any worse than Maupay.
Might be a way of getting him involved more.