Hi Heather,
Have you tried enabling guest access to courses (via the course settings) without enabling guest access to your site i.e. keep the guest login button hidden? This will allow logged in users to access other courses without needing to enrol.
You can check out this feature on moodle.org by visiting another course which you're not currently enrolled in, say the Moodle Lounge. You'll notice in the course admin block that you are given the option to enrol in the course.
Enrolment
accessing a course without enrolling into it
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