Is it possible to access the app data of third party apps that aren't listed under File Sharing in iCloud?

I have an iPhone 6 Plus with a large library of photos stored in a somehwhat obscure third party photo frame app. The app imports photos from the photo library and saves its own copies to display in a slideshow. The owner of the device deleted photos from their library after adding them to the photo frame. As a result, the only place these photos are stored is in the app data of this somewhat obscure third party photo frame app.

I am trying to retrieve the original photo files from the app data. The app is backed up on iCloud but doesn't list its app data under File Sharing in iTunes so you can't access the data.

Is there a way to access the files stored in app data? I've already tried this terminal command:

ln -s ~/Library/Mobile\ Documents/ ~/Desktop/Real\ iCloud\ Drive

to show hidden folders in iCloud Drive - this shows some apps and their data but not the photo frame app.

Can I somehow download a copy of the entire iCloud Backup and navigate to them that way?

EDIT: I'm thinking accessing via iCloud because the files are stored there but of course they are also on the device itself so if it's more feasible to do it that way then I would love any suggestions for that too

You could try backing up the iPhone in iTunes and dig into the backup locally to see if the photo files are in the app's backed up data. I work on a tool that can help you dig into the backup files, Decipher Backup Browser: https://deciphertools.com/decipher-backup-browser.html

You could either look through the app's data in all files view, or try the all photos gallery view in DBB. Or if you want to contact me directly about searching around the backup, there's a contact link at the top of all of our pages.