Hello @Anonymous,
If you have 100,000 premium licenses for the Power Platform, adding an additional environment is probably not going to bust your Dataverse resource capacity, so if you feel better about storing non-core CoE components in another environment, why not?
Still, your 100,000 makers will probably still have to access your CoE environment, e.g. to access the Developer Compliance Center.
And I disagree regarding the Theming components: the final products will not need access or be located in the Dataverse used to host the Theming components. Instead, you use the Theming components to create an .msapp file that is used as the basis for creating a canvas app with your corporate identity. This file can be downloaded to a local computer and for example shared through a SharePoint Online hub site for the Power Platform.