On the local payment methods point, that is the part of this thread I would act on first, and it has an awkward consequence for measuring any of the rest.
One correction to something assumed further up. The suggestion to open each abandoned checkout and record the country, method and decline message by hand is more work than it needs to be.
I went looking to confirm that Shopify does not expose this in a structured form and the schema says otherwise.
The Admin GraphQL API carries an OrderTransactionErrorCode on the order transaction, 47 values, and it is already close to the by-next-action split described above.
DO_NOT_HONOR, CALL_ISSUER, INSUFFICIENT_FUNDS and PROCESSING_ERROR sit apart from EXPIRED_CARD, INCORRECT_NUMBER, INVALID_CVC and INCORRECT_ZIP, and apart again from CARD_TESTING and FRAUD_SUSPECTED.
AUTHENTICATION_FAILED and AUTHENTICATION_REQUIRED are their own values, so a 3D Secure failure is already distinguishable from an issuer decline without inferring anything from a message string. It is a field to read rather than a taxonomy to build.
The 3DS drop-off is the real hole and it is worse than a reporting gap. There is no value in that enum for a shopper who starts the challenge and leaves, and there cannot be one. No transaction is created, so there is no row to carry an error code.
That state is missing from the data model, not from the reporting, which is why it does not appear however hard you look for it in Admin. On Shopify Payments there is no separate gateway portal to recover it from either, so for that setup the number does not exist anywhere rather than existing somewhere inconvenient.
Which is what makes the local methods argument hard to prove. Moving a Dutch buyer to iDEAL removes a 3DS challenge that would otherwise have had a chance to be abandoned.
The gain is real and the one measurement that would demonstrate it is the one that cannot be produced.
So it will not appear as fewer 3DS failures. It appears as authorisation rate rising with no matching fall in any decline code, and anyone watching the decline breakdown will see the improvement arrive from nowhere and attribute it to something else.
Worth knowing before that happens rather than after.