Exceptionally large clumps might be a concern, but I'd also be a bit concerned about predictable patterns or general movements of cards throughout the deck.
I think that can be largely eleminated by using cutting and stripping (sometimes called side shuffling).
The new wallet wizard could walk the user (possibly with diagrams and/or video) through this process (even if starting from a pre-determined new deck of cards).
Cut the deck randomly.
Riffle shuffle 1 time.
Strip shuffle.
Riffle shuffle 2 times.
Strip shuffle.
Riffle shuffle 3 times.
Strip shuffle.
Riffle shuffle 4 times.
Cut the deck randomly.
Flip the deck over and record the cards in order into the new wallet wizard.
Would it not be simpler to throw the cards on the floor, take a picture and then get the SHA256 of the raw photo bytes ?