Just checked out Dynamix’s guitar strummer and Power drums with Bass for PSR-S970. Sounds promising as guitar strumming is somewhat weak on 970. Power drums seem to add some nice punch as well. They offer strums with drums and bass for roughly $150 and strums only for $80?

They say they work on all Yamaha keyboards.
JM
https://youtu.be/g0oByjHGSLI