Thank you so much for the detailed response, and listing off all the steps involved in making N10.3 stable with 10.15.5…Really appreciate it!
Resetting the SMC - yes, this was done once, but perhaps should have been repeated. Yes, I read about deleting the preferences folder and this step was also performed, and then launching N10 with no 3rd party plugs. However, the plugs folder was not renamed…
Agreed - Catalina seems to have a lot of issues that have been observed in other posts. 10.16 beta is now available for installation, but I’m tentative to say the least…
For the time being, I’ve installed N8, which has proven thus far to be rock-solid, though definitely not as pretty as N10.3. 
I’ll give 10.3 another go next week when I have more time to tinker - following all the fantastic advice you posted. Much appreciated, and hope to report back that all is sorted…