I would certainly buy a day pass on the day of travel as I did when I
was in Germany in October. I’m just trying to figure out if it would
cover Heidelberg. RMV doesn’t.
Neither does the Hessenticket. But the link I gave you for the H'ticket indicates how close you can get to Heidelberg with the H'ticket. It is valid for travel...
"...in the cross-network fare zones between VRN and RMV
to the towns of Hohensachsen and Lützelsachsen in the township of Weinheim..."
Which means you'd need a supplemental ticket for Weinheim-Lützelsachsen to Heidelberg Hbf to stay on the train all the way. On the RB train from Frankfurt, that's about 30 additional travel minutes through the VRN - and you'd need a VRN ticket for that stretch. OR, as you suggested, perhaps the Day Ticket for Germany.
Hessenticket map: https://www.vrn.de/mam/tickets/tarifgebiete/hessenticket.jpg
Can't help you with what's on the DB app and what isn't.
As Sam indicated, the RMV Day Ticket has a generous travel zone with many fine places to visit. The same is true for the Hessenticket. You'll probably have a very nice day staying within those boundaries. Heppenheim - accessible on that same RB route to Heidelberg - would be a nice trade-off, for Heidelberg, just for example. Both the RMV and the Hessenticket passes should get you there.