Update:
Complete raid wipe and rebuild, along with a complete reinstall of both hosts yielded no change. The error I get when I try to create an SR from the CLI is a bit different than before, though;
xe sr-create name-label="iSCSI DRBD Storage" host-uuid=e3589379-6735-490f-a4af-e9213ccb6d1d type=lvmoiscsi content-type=user device-config:target=10.100.4.1 device-config:port=3260 device-config:targetIQN=iqn.2018-01.com.lstaff:srvpool1 device-config:SCSIid=360000000000000000e0000000001000f
Error code: SR_BACKEND_FAILURE_47
Error parameters: , The SR is not available [opterr=Logical Volume unmount/deactivate error [opterr=errno is 5]],