Something like "allow app access to last photo" would be ideal for me

Why?

If they have access to the last photo ... every photo you ever took was the last photo. Why mess around giving app's permission to surveil/siphon off your photos at all?

Any carte blanche for apps, and apps will go to great lengths to take advantage of that in unexpected ways, and obscure the fact they are doing so.

And privacy losses can never be verifiably reversed.

All most apps need is for you to select photos to upload/import using an Apple supplied photo selector. So they only see and get exactly what you want them to have.