How to handle custom text on a channel index page?
I'd recommend to either give each channel's index page a dedicated single section or you configure a structure section to contain them all. An advantage of using a structure over separate singles is that you can generate the site's main navigation from it. And you can add a hierarchy to it.

The structure settings in the CP:

URL for Top-Level Entries:
{slug}

URL for Nested Entries:
{parent.uri}/{slug}

You probably want to give your index pages individual templates. To do this just add multiple entry types to your structure and relate each entry type with a template as described in the FAQs.