Tagging @jojo12100 as well.
So I was recently able to recreate the code for encoding/decoding ereader cards which allows us to see the actual trainer and pokemon data very clearly and I was able to dump the data from all the officially released cards.
Next I looked into those 21 cards that can be found in the colosseum ROM (contained in menu_carde.fsys) but the format for them is slightly different, the format must have changed a little by the end of development. This means, in their current form they didn't work with my code, nor do they work if you force load them into the game for the game to decode. Whoever said the thing about the 21st card containing "Team Snagem Goku" is probably making it up.
However, I was finally able to decode those cards yesterday. It required a couple of small changes to the decoder which I was able to figure out with a little trial and error. I've dumped the data from all 21 cards but there's nothing super interesting on them unfortunately. They're all test data that was automatically generated by an algorithm; they were not in any way designed or intended as game content. They were only for the purpose of testing the ereader functionality and are unrelated to the officially released cards.
In case anyone is curious anyway, most of the cards contain data for shadow makuhita, misdreavus and spoink with shadow ids 69, 70 and 71 respectively. The last card has data for shadow regirock, rayquaza and deoxys but only because the algorithm filled the cards in pokedex order and those pokemon happened to land on the shadow slots on the last card.