\Arrives from the kpop fandom**
I have this A5 binder and the WITHYU 4 pocket pages for some of my photocards. I grabbed some of my magic cards and they fit just fine. Even with having penny sleeves on them they still fit without any problems.
I think as long as you check that the pocket size is 2.5" X 3.5" when buying pages you'll be fine. Amazon has a few A5 binders. Etsy is a really good place to look though. I know the composition style are getting popular with photocard collectors. The Be on D binder is probably the most popular. Something that is a bit harder to find but growing in popularity are the A5 wide binders. They're made for 6 pocket pages.
Just keep in mind that many A5 binders don't hold that many pages so if you don't want a million binders around, check to see how many pages the binder holds. You'd be surprised how quickly they can fill up once you start putting cards in them. If you have a lot of cash to drop though, you could check out Prism Platinum. They're in Australia so the shipping is also pretty expensive but they've made some 3 ring A5 deluxe binders that hold a decent amount of pages. Sometimes you can find sellers on Instagram that'll hold group orders for them to help cut down on shipping costs.
As for A5 paper, I've never used it so I can't really help you with margins. You could try buying it pre-punched and then measure it yourself to see how much space you have for graphics. The 4 pocket pages I use say they're 6.2" x 7.8" though so the paper and the pages won't line up exactly if that's something that'll bother you.