Snapseed

Lucy ~ A simple custom shortcut created in Apple's Shortcuts app will do what you want. The shortcut below copies the photo being shared to the clipboard and opens Snapseed. Then in Snapseed tap OPEN at top left and then tap Paste image at the bottom. If this solution interests you, you can either build the shortcut yourself, or I can post an iCloud link to it. ...But perhaps Snapseed's Help will offer an even simpler solution.

Shortcuts User Guide for iPhone and iPad

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Okay Lucy, when you tap the iCloud link below, it will open in the Shortcuts app. Tap the blue "Add Shortcut" button at the bottom and the shortcut will added to your shortcuts collection. Then when you share a photo, you should see "Open Snapseed" in the list of options.

By the way, when that share sheet appears, the list of actions can be edited: Scroll to the bottom and tap "Edit Actions..." You can add the shortcut to Favorites and/or reorder the list. If you have any problems, let me know. Here's the iCloud link to the shortcut:

Open Snapseed

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Reach out to Snapseed Help => https://support.google.com/snapseed/answer/6182613?hl=en#zippy=

Thanks for your feedback & you're welcome.