If DL is not publishing a fare for GRR-ALB via ATL that is a conscious decision if it did so in the past. Likely a poor financial decision for DL, so it dropped the fare.
Do bear in mind that you do not have a connection if you are on separate tickets and that if you need to make a change or cancel, you will need to pay a change fee for each ticket (typically $200 for a standard domestic penalty fare). Thus, if your plans change by a day, it will cost $400 to make the change.
For what it is worth, I am seeing a mix of, -900's, -700's, and a -175 on the route. Those are all 2-cabin aircraft with operational parameters at least the same or exceeding DL's typical mainline narrow bodies. The three aircraft also have a more comfortable Y cabin (to the extent any Y cabin is comfortable) with 2-2 seating. Thus, I am struggling a bit as to why anyone would fly 1,000 miles out of the way to do this.