How can I block user from viewing issue tickets unless they are assigned or a watcher?
You can add this restriction to issues using an Issue Security Scheme, see https://confluence.atlassian.com/adminjiraserver073/configuring-issue-level-security-861253265.html
i would say use the Issue security level it will control which user or groups of the user can view the Jira issue.
Please have a look at the below KB, hope it helps.
https://confluence.atlassian.com/adminjiracloud/configuring-issue-level-security-776636711.html
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