Welcome to the forum!
You said the word "snap" in your message, but I'm not clear if you've actually discovered the feature called Snapping. If not, that would be a good thing to learn.
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Snapping.htmlThere are probably several ways to do what you're asking. From what I understand so far, I would use Guides. Or you could use a grid as well. Have you discovered the round corner option for the Rectangle tool?
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Shapes-Rectangles.htmlI would consider creating a custom template, with the page borders set for the size of each card, and which has the guides in place already. Or you could do it all in one file, and just move objects in and out of the card area, if you're going to export PNGs. I guess it depends on what you're going to do with them, and what file format you need. Do you want to end up with one file which has all the cards in it? Or do you want one file for each card?
I would line up a vertical guide along each side of the card, which would be the centerline for the card number and suit icon. I would probably also make 2 horizontal guides (top and bottom), where you can align the suit name. Maybe a vertical guide at the center of the card too. When you type the text, be sure that you make the text centered. That will place a text anchor at the center point of the text, which can be used to snap the text to the guide or guides intersection. And as you'll learn, for the suit icon, there's a snap to center option.
The opportunities with guides and snapping are endless! You can even rotate guides and give them different colors! Since it sounds like you're a bit of a self-starter, I think I'll stop there, and let you ask for whatever you need next. Here's the whole manual chapter on Positioning and Transforming:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Position.html I'm guessing the info on Guides is going to be in there, but just in case, here's that link:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Snapping.html#Snapping-GuideLayers could be used, but personally I probably wouldn't need them. I typically use layers for very complex drawings, to help with selections. But I suppose I could see a case for each card having it's own layer, and then you just hide the layer when you finish one card, and start a new one. The guides and page border would remain in place for all layers. But that kind of depends on what your end goal is - a file for each card, or all the cards in one file, or whatevere you're going to do with them.
It sounds like a good approach to group the number and suit icon, and then duplicate and rotate for the other corner. However, I would use the guides for alignment, and keep the rounded corner attached to the rest of the card. As you'll see, snapping is like magic! The guides will ensure that the number/icon pair goes in the right place.
Or with proper placement of guides, you could create guide intersections for each individual object. But this is getting more into individual preferences. And you'll develop your own.
Let us know what else you need