How can I lock specific content in my shop and only allow my subscribers to view it?

You could probably achieve this with our feature, which allows you to tag your subscribers with an active subscription and in combination with an app that restricts content based on the customer tags. This can be turned on in Seal Subscriptions -> Settings -> General Settings -> Customers.
This way, the app will tag customers that have an active subscription with a specific tag.

However, you would have to find an app that allows you to unlock content to customers that are tagged with a specific tag.