Foyth needs to play in my opinion. Yes Tanganga has showed that he can be included as well, but there is no reason why Foyth and Tanganga cannot compete for minutes over the rest of the season. Dav is clearly our best CB right now and needs to play every game, which is great for his development too as he is still young.
The Jan / Toby situation is clear for us all to see now that they are only a shadow of their former selves and we would be hurting our long-term prospects if we continue to overplay them. I honestly can't see how Foyth would defend worse than either Toby or Jan if given a chance, as they are both painfully average (or worse!) right now.
Toby obviously has his signature long-ball ping which is keeping him in a job at the moment, but how many times already have we seen him being dribbled at by attackers only to retreat side-on and allow them to shoot or create? This is not anywhere near the level of defending we used to get from one of the best 1v1 defenders in the league.
Foyth, Tanganga and even Davinson will make mistakes too, but they need to make those mistakes in order to learn, and not simply watch our veteran CBs making mistakes and go unpunished by selection.