Creating custom entities are really useful for exposing attributes or collating data from multiple entities into one useful one for automations, however there is no way to associate these sorts of entities with the device which they are related to so that they may show up on the device’s page.
What would be fantastic would be to be ability to assign the a device with an entity (be it a template, sensor, or input entity) and to also specify if it should be place in the “control”, “diagnostic”, “sensor”, “configuration” and additionally a new section “custom” which would be used for unlabelled entities that have been associated with a device.