Is there a way to view details regarding file sharing activity? The activity app does show the sharer and sharee when
A file or folder has been shared
A new file or folder has been created
A file or folder has been changed
A file or folder has been deleted
but is there a way to see who accesses a file? I suppose one can look at the logs but that would be a bit cumbersome.
For personal use, the following suggestion may be overkill.
Installing a Splunk server and the Splunk App for Nextcloud (https://splunkbase.splunk.com/app/3398/) can do exactly what you are asking (and a lot more) by analysing the logs for you.
Splunk looks very interesting although, as you say, may be overkill for personal use. I have 2 questions. does Splunk require that your data leave the NC server and thereby create a privacy issue? Also, what does Splunk for NC cost?
There is an NC app called Activity Log but to my knowledge it has not been updated since v11.
Re. data leaving the NC server: To make use of the Splunk App for Nextcloud, the logs themselves are ingested and stored by the Splunk server. Splunk comes with the usual RBAC support you would expect to secure who can access what. The privacy issue is no bigger than the issue you have with the data stored in the Nextcloud logs themselves.
Although I work for Splunk, the App is a hobby project of mine. If you were indeed interested in a Splunk license you need to go to the Splunk website to get contact details there.
There is a free version of Splunk available which can ingest up to 500 MB of log data per day. After 60 days RBAC is however disabled, so accessing the server after that is not secure. If it is installed on-prem in a small environment this may of course not be a major issue.
So just to be clear, if my logs are not > 500MB /day (likely) then the server is free and the data is never leaving my domain? My NC instance is using docker containers so is there a (relatively) easy way to get a containerized Splunk server up and talking to NC?
That is correct, you can ingest up to 500 MB of new log data every day for free. But as I mentioned above, after 60 days, some functionality disappears. Since this is a Nextcloud forum I probably have to limit how much Spunk support I provide 
Splunk IS supported inside a docker container. I have never run Splunk in docker myself so I don’t know how easy or difficult it is to set up. Some of my colleagues do it. I would take a look here: https://splunk.github.io/docker-splunk/