Definitely possible but unlikely. From the sound of it, he can agree to a deal with the Sox for the 2022 period and spend this year playing/practicing at one of their facilities.
Agreed about the backup part but I think myself and others wouldn't mind him in a DH/rotating IF role where he could give Moncada at rest at 3B. I think its likely he gets a starting position in the field somewhere though.
DH and rotating around the infield I would think. But I think you're right. I think he signs somewhere to start at a position rather than mostly DH.
Edit: you can give Moncada's legs a break and DH him once a week.
I mean, he could sign with whoever has the most remaining. It just wouldn't be an amount similar to Cespedes. Personally, I think he should sign in this period and be with a team but we'll see.