I think you are looking for an image editor for graphic composition, when you want to add text or shapes, or want to combine several photos into one. Pixelmator is a great tool for this kind of things. You mentioned "simple adjustments", but the term "adjustments" is usually referring to developing an image to enhance the lighting, the color, the contrast etc. This can be done easily in the native Photos.app on Apple devices, but not graphics composition. For this we need to call extensions from other apps.
Mark has already pointed out Apple's own Markup tools extension - they are available in quite a few Apple apps, in Photos, in Mail, in Preview.
When I want to add Graphics to a photo I am simply using Apple's iWork apps - Pages or Keynote. Keynote is great to add shapes and text to a photo. I just add the photo I want to modify to a slide in Keynote and then add text blocks or shapes.