I stuck with Mojave for ages mainly for Aperture and iPhoto, a great combination. And I could use external editors if needed. My wife still uses Mojave, for iPhoto and iTunes. And Firefox ESR, but eventually she'll have to upgrade.
I upgraded
yesterday, to Sonoma. My phone use .heic images and hevc videos, and it was just too much hassle not being able to use them in iPhoto.
I had tried Photos in Monterey, didn't use it much but just tested all images were transfered ok from iPhoto library, and I got the impression I could use external editors by "dragging" images from Photos to, say, GraphicConverter.
But in Sonoma I can't find ANY way to edit images from Photos without an automatic conversion to .tiff, and when I try to "drag" to another app, the images are sometimes automagically and randomly (?) converted to jpeg first! Either way I lose metadata or quality. I want to use an external editor to edit the original file, is that impossible or am I just a Photos noob?
I have Pixelmator Pro (Photos converts to .tiff), Affinity (Photos converts to .tiff) if I open in external editor.
If I "drag" an image from Photos to Affinity or Pixelmator Pro I sometimes get the full image opened in .heic. or jpg, sometimes I get a lower resolution image opened in .jpg. Same camera, same album. I'm bewildered.
Anyway, If they could integrate Pixelmator into Photos it might work out... Photomator seems to be interesting, but I haven't tried it yet, it's expensive (already paid for Pixelmator Pro) and I'm not sure how it integrates with photos.library.