I think there are definitely ways you can make your life a little easier.
A way that looks quite simple (but I haven't used it myself or tested it) - would be to restrict the topic using this plugin: https://moodle.org/plugins/availability_days - I think it would work if you used enrollment start dates correctly. So section 2 is available 7 days after a user's enrollment starts, section 3 is available 14 days after, etc.
Another way, without using plugins, would be to use nested restrictions (restriction set) for each topic. You would still have to go in every so often as new groups come in or as you re-use groups but I think it would cut down a lot of your admin. See screenshot attached.