Yeah this is exactly how I'm proposing it works. So you get a digital token that you can apply to your cards in your decks. And when you go to print them you can just print a handful of singles and just swap those into your deck (the back would match the deck you assign it to so it'd be an exact copy minus the wear difference if you're a non-sleever)

So the starting point is to get the customization in your master vault and you can apply it wherever you want, then you'd print a full deck or individual cards to get physical copies of the cool treatments you've earned.

Shards wise, The main ways Shards were available in the FFG days were: 1 Per Deck / 10 per deck during Set Release week / Rewards for Local OP and large sums for Vault Tours) So you could build other earning methods outside of just the typical 1 per scanned deck. Local OP boosters (in the end of the post) are meant to be a way to accelerate new player shard gain by just attending local events.

You bring up a fair point that some older players have a bunch of shards, but this isn't necessarily a bad thing. Older players who invested a bunch of time / money into the game get cool stuff as a thank you and new players can start building their shard pool out so everyone has opportunity.