Before anyone suggests using alternativeto.net, I checked over there but all it is showing are the usual document scanners. While both are similar, Photoscan deals with reflections better because it takes 4 different photos from different angles to the centre of the image and then merge it to give a clean scan of the image. And I couldn’t find any FOSS alternative which does the same.
I’m not aware of of an alternative yet. I have a pal who is excellent with GIMP as he enjoys his astrophotography where he produces super clean images from multiple exposures of the stars. He seems to be able to use GIMP the same way as Photoscan, but simply uses Photoscan as it’s just simpler. I haven’t taken the dive into GIMP as there’s that learning curve, but maybe need to bite the bullet for the times I need it in the meantime.