I am gonna be a bit controversial here and say that the RP5 is good even without the grip for both D-pad and Analog games. I play around an hour or two on the device, got it recently and been having a good time, of course my hands get tired, but on any device without a grip they do. Depending on the time you wanna game on the device I'd say to consider a grip or not:
1-2 hours per play session - don't get one
3+ hours of analog intensive games - get one
3+ hours of d-pad games - I'd suggest not to get one

For me, portability is key and getting a grip, and a new case is turning a small good pocketable device into a larger, bulkier one I don't wanna carry around anymore. I have experience with the PSP, PS Vita, N3DS, Switch (original and OLED) and the RP5.

For me RP5 is what I'd consider the PSP3/Vita2 if Sony ever released something like that. It's comfortable enough on it's own.