Nobody is saying Devers or Casas should be part timers. Casas could "fit" at DH or half-time DH and half-time 1Bman.
As we know, Cora loves to rest players 8-12 games a year. It might be nice, if we rest Casas and Devers as a DH some of those days, so we keep their bats in the line-up, more often.
When Casas is resting, either on the bench or at DH, I'd like to see Devers play 1B, after he gets some reps in ST'ing. Nobody is missing PAs with this idea.
The idea of trading Yoshida seems remote, to me. For the same reasons you want him gone, other GMs don't want him and his salary. We'd likely have to pay some of his salary or take back a big contract in return.
I'm just going by what we have. If we trade Yoshida, then it makes more sense to DH Casas and Devers more often. The idea of playing Casas and Devers at 1B and DH, only, makes some sense, assuming we add a 3Bman or trust Urias to be the FT 3Bman. Our D would improve at 3B and 1B, and no PAs would be lost from Casas or Devers.
If we trade Dugo, and don't add an OF'er, we might see this:
LF- Yoshida (Duran/Ref)
CF- Duran (Rafaela/Abreu)
RF- Rafaela/Abreu
This would also free up the DH position.