I have no idea if this will be useful for you.. but I had fun playing with it.
I saw your post and recalled something I read about a few weeks back..
I feel like it could be a pretty powerful tool for ya, right on the iPhone.
So give it a whirl on a small sample, see if you can scale it to your needs?
I'm using iOS 14.4
- Go to the Photos App and create a new album called Converted, don't select any photos, just hit Done
- Create another album called NotConverted, select a dozen or so test images (HEIF)
- Go to the Shortcuts app, select the + sign in the upper right to create a new shortcut
- in the search field at the bottom, type 'Find Photos', and select that item under Actions.
- Back on the New Shortcut screen, if you tap Add Filter, should have the 'Album is Recent' pre populated. Tap on Recent and select NotConverted. Now the filter should say 'Album is NotConverted'
- Add another filter just in case you added Videos to the Convert album.. Add Filter, tap on Album, select Media Type from the list.. it should now show the added filter as 'Media Type is Image'
- Tap on the white + in a blue circle below the function block to add a new operation.
- Type 'Convert Image' into the field, tap on the result under Actions. Now you have 2 functions in the shortcut. It should automatically link the previous operation and automatically populate with 'Convert Photos to JPEG'
- Tap on the + below the function block to add a new operation. Type 'Save to Photo Album' and select it under Actions
- It should auto populate with Save Converted Image to Recents.. Tap on Recents and select Converted. It should now say Save Converted Image to Converted
- In the upper right select 'Next', name your shortcut.. 'Convert Test' or whatever you want. Select Done
- Now you have a shortcut tile in My Shortcuts. tap on the tile to run it.
- If all went well, you should have a copy of each image in the NotConverted album in the Converted album that is a JPEG.
I'm not quite sure what it'd do if you used a filter of Album is Recent.. If all 15k of your images are in there, who knows how long that'd take. Also have to consider that this is duplicating images, so it'll start taking up ~twice the space on your phone. You might be able to add something to the shortcut that deletes the photo as it converts it?
If you feel like playing more, you can replace #4 with 'Select Photos', set Include to Images, turn on the toggle for Select Multiple. Then when you run the shortcut, you can go browse for photos to convert. It doesn't give you the option to Select All, but you can tap and drag to save multiple photos. You can even drag off the screen slightly and it'll continue to select as it scrolls.
