I'm specifically asking about a P420 in a D380p G8 server. I'm planning on using storage spaces direct requiring a simple HBA. So I've disabled RAID on the P420's. I'm curious if in this JBOD mode the cache on the raid card is even used?
All on-board caches are completely bypassed in JBOD mode. If you really need to use them you might want to look at a single-disk RAID0 config, which is unsupported but works with S2D as long as you patch "RAID" -> "SAS" BusType in registry.
(That's for VMware VSAN but hardware config and approach are same...)
In general if you want to get some use out of the hardware you already own you'll have to use software-defined storage stack with zero dependency on underlying hardware. Look at HPE VSA and StarWind vSAN, both have free versions to experiment with.
What you normally do - you combine local hardware RAID with inter-node replication for performance and higher level of fault tolerance. This approach is widely used by HPE VSA and SimpliVity (now part of HPE as well) HCI.