CardcaptOSR Sakura
When I wrote up all of the Cardcaptor Sakura material I've recently posted, I had mostly played and ran 5e, so all it was made for that system. I was aware of older editions and had poked my nose in when it was relevant to games I was running (in 2019 I ran Ghosts of Saltmarsh, set in the Sea of Fallen Stars, so there was a lot of looking at older D&D editions for any material on the area), but it wasn't until a few years later when I was running a Dread Domains hopping Curse of Strahd game that I really did much looking into older editions, and wasn't until I ended up on Bluesky that I really got exposed to the wealth of OSR blogs and material that's out there.
Ultimately, Cardcaptor and 5e just don't gel well together. There's nothing wrong with that, but like many 5e players I was trying to make a square peg fit a round hole. Trying to shove Cardcaptor into a more OSR game model might be the same, but the core concept of the show - Sakura collecting the clow cards fits better into OSR paradigms than it ever really will into 5e.
It's important to note that I'm far from any kind of expert on the OSR. I really just enjoy reading about it - I've yet to actually play in a game or run a game myself in any OSR system. I'm working on some solo gameplay stuff, but if any table in the Las Vegas area is looking for a player, let me know.
Advancement through inventory
Ultimately, Cardcaptor Sakura is a story about the protagonist collecting magical items and becoming stronger through that. While Sakura's magical capacity and energy grows throughout the series, it's not the same way a wizard or sorcerer in 5e is - she doesn't start with little gusts of wind to hurricane force winds. It's the same "spell" of using the Windy to make chains of binding winds that she uses throughout, she just gains more cards to use as tools as she goes. The Fly lets her move around in different ways, the Power lets her move things around, the Mirror lets her leave duplicates of herself to keep her brother from noticing, etc...
Having a variety of cards to choose form, alongside the wand, makes more sense in a more item/inventory focused system than 5e. Especially when you're not dealing with attunement and limits on how many magic items you can be using at a given time, like in 5e. The Clow Cards feel like they'd be fun for a system like Cairn (although unless you're doing some similar inventory shenanigans as I did you'd run into that problem), since it would give a variety of abilities for creative problem solving.
Creative use of abilities
You've also got a wide variety of abilities in play with the different cards. Especially if you're using the full 52 card set from the anime instead of just the 19 cards from the manga, it's a lot of design space to cover, with a fair amount of overlap. What exactly makes the Float different from the Move? The Arrow from the Bolt? If you're using a more open ended system, you've got more room to differentiate the effects than just the existing abilities.
Even just the existence of reversed spells makes thing a little easier. Part of the gimmick of The Light and the Dark is that they are equal and opposite, but the 5e spell list really doesn't allow for that. Being able to say one card just does the same stuff as another, but reversed makes things easier.
You've also got interesting decisions to make in terms of distributing the cards. Do you give the Sword to the fighter, to guarantee them a magic weapon? Or do you give it to the magic user so they've got a backup weapon when they're running low on spells? Does the fact that certain cards overlap allow you to spread them across a party, or focus them on one primary cardcaptor? Depending on how the mechanics of the cards work out, they'd be an interesting set of items to distribute across a party for problem solving purposes.
Yue as a Follower
As I mentioned in the second post, while Keroberos works as a familiar, it feels weird to treat Yue/Yukito that way. He's an angel that spends most of his time as a regular dude - making him a familiar feels weird. Having him as a special follower would make more sense.
Class Issues
As I mentioned in one of the posts, in 5e terms Sakura would be a sorcerer - her magic power is innate and she's just channeling it as she develops it, with Clow Reed fashioning the cards, the rod, Kero, and Yue as tools and teachers he thought would appeal to her. But the style of casting is very much wizard - finding additional spells through exploration and eventually developing her own spells. Sakura's powers just don't quite map onto how 5e has divided magical powers, but with the more barebones class chassis in the OSR it works out fine. The closest things to class abilities Sakura develops are her divination skills and her ability to sense the cards and other magic. Some kind of custom spin on the Acolyte/Mage idea from Old School Essential Carcass Crawler 1 (a magic skill based class that uses scrolls instead of spells) would probably fit best, but the more barebones classes of OSR would work better than trying to make cardcaptor a subclass of one of the spellcasting classes in 5e.
I think it would be interesting to see what an OSR Cardcaptor would look like, so maybe I'll take a stab at it at some point. But I'd love to hear other people's thoughts on it too!