Now allowing me to add extra field - InvalidArgumentException: Field field_test is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 587 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).

Can you remember, how you've fixed the problem?

For anyone else landing here - in my case I was missing the "bundle" entity key in the class annotations of my custom entity class... I was originally planning to write a bundleless class, but switched then to bundle-support and missed out this single line

Thank you, agoradesign, because your #3 answer helped me! And I was exactly in the same situation as you.

you're welcome, glad that I could help :)