Since you can edit photos from the Photos app in Pixelmator Pro and then save them in such a way as to retain the layers, you can use this to keep multiple variations of your photo and other elements in a single slot in your Photos library.
Note: This is Club MacMost exclusive content. To view this video click the thumbnail below to go to the post as Patreon.com. Once there you will be able to view the video as long as you are logged into your Patreon account and a current supporter of MacMost at the "Club MacMost" or higher level.