Story should still be a plus defender at SS for a few more years, even if his arm is weakening or getting less accurate. I'm not sure Mayer is a better defender, now, and until he is, I'm keeping Story at SS, when healthy, and assuming he's not falling apart.
I think Mayer could be a decent SS, now, and maybe would be better at 3B than 2B, but he should be fine at either, when that time comes. Campbell looks like he should be our 2Bman, but he may end up in the OF, but that choice is for later on, once we see how Story looks, and who looks ready first: Campbell or Mayer.
Bregman at 3B vs 2B is also a side issue, but I think we keep him at 3B for as long as he is with us.
If everyone is doing well and healthy, and the kids have proven they deserve a FT shot, the only way I see them all playing is with campbell in the OF, but that could be a long time coming, and everyone healthy is a big if.
Maybe this:
DH: Devers
C: Wong/Narvaez
1B: Casas
2B: Mayer
SS: Story
3B: Bregman
LF: Campbell
CF: Duran
RF: Abreu/Refsnyder or Anthony
Rafaela as the supersub
Others on bench: Yoshida and DHam