Where do I store custom values/content?
I am not sure about what is the question here. In general:

If you want to store content, create a ContentEntityType that extends ContentEntityBase. If you want to create a configuration, create a ConfigEntityType that extends ConfigEntityBase.

What you need sounds like a ContentType where you create your main content by code and have another Form to add/modify the content of a "page description" field in the ContentType. Maybe you can also use "Node" that could save a lot work.