How much "CRM" is relevant to single pilot ops?
Plenty.
Situational Awareness
Threat and Error management
Communication (causal factor in 70%+ of all accidents)
Management of emergency situations
etc. etc.
Other than the fact that you can't have a fight on a single pilot aeroplane, you have all the problems that any multi-crew flightdeck will have, just without the ability to share the workload.