Hey everyone! Thanks for all the great information. I'm not adverse to using an external app for scans, as long as it's easy to load them into Photo. I do have the software that came with my Canon Canoscan 5600, but it's clunky and not the easiest thing to use in terms of getting files into the editing software. I guess I just got lazy using the TWAIN plug-in to load the scanning interface directly into Photoshop. But if I go with Affinity Photo, I guess I'll stick with that -- unless Canon stops publishing updates for new Apple OS releases. In which case I suppose I'll need to switch over to something like Vuescan.
Again, thanks to each of you for the quick replies and the detailed info. You've helped me a lot in understanding my options as I try to find alternatives to the subscription model that Adobe has foisted upon us.