Are there migration plugins for D8 sources? I found 2 references here for migrating D8 to D8, but both are very outdated. 

Certainly there will be a need to do D7 -> D9 or D8 -> D9 migrations for many, but there are also current use cases. One is migrating from a multisite to individual sites. 

Mine is the site restructuring its content types, and it would be far simpler to add custom code to a source plugin than to do all of it in custom code.
Yes there is. Drupal core has a content_entity source plugin that's a part of the Migrate Drupal module. 

The one caveat with this plugin is it can only handle scenarios where the source/destination DB is the same, which sounds like your scenario here.

If source/destination DB isn't the same, the Drupal 8 migration provides a d8_entity source plugin that does support this scenario.