How do you delete every object but ones from a group in the delete DOP?
I tried: “!group”, with no success.

In the Delete node: Operation/“Delete Non-selected” & have the group selected in Group

Edit: just saw you need Dops, not Sops.

you can use * ^, which literally mean all(*) except(^)
* ^objname
or
* ^objid
or
* ^groupname

Thanks, I used to know all of this, it's been about 3 or so months since I've really used it though, can't believe I got ! confused with it.

How do you make it re-evaluate every frame? Currently because no objects are selected it deletes all, but as objects start to group it doesn't update.

it works for me
each time some new objects are added to sim, only objects with name piece1 will be kept (* ^piece1)

or post your file if you have specific problem

As you can see in this file, deleting group works, but deleting all but it does not.

it works correctly, but you need to keep in mind that DOPs are simulation environment so here is what happens in your file
frame 1
- you create all objects
- group will not be created since the condition is not satisfied for any piece
- delete will delete all objects except group so except none so all objects
frame 2
- no new objects are created
- there are no objects actually left in sim so nothing to group
- nothing to delete
all later frames the same case

you can use your way in SOPs and feed just objects that need to move to DOPs
or just split pieces in SOPs after your sim like you meant in DOPs