Sorry GB, but I'm in total disagreement
... but we've already got rid of the Brevet Card in some Audaxes (e.g. DIYxGPS) and those rides certainly count as Audaxes, so that boat has already sailed.
GPS validation for DIY Perms works because a DIY perm is a personal event administered jointly by the rider and AUK, and DIYers follow the prescribed procedures and take pains to use their GPS devices correctly. Consequently problems are generally few and far between which means we have the capacity to help resolve them. Also, as they are personal events there is relatively little scope for fraud. Once you have a large number of riders completing the same track to the same schedule that's not the case.
As AUK cannot/prefers not to invest in current commercial timer/dibber solutions, the way to go is 'smart gps' devices which can provide encrypted tracks and/or (more likely, I expect) a ubiquitous smartphone App which can register and track riders as a group. Much of the plumbing for both of these solutions already exists one way or another already albeit not in a readily usable format and history suggests it will be some time yet before a solution is available and adopted.
(I'll open the betting at 3-5 years, i.e., once the new AUK website which any such solution would have to talk to is bedded in).
So rather than the boat having sailed it's rather still on the architects sketchpad pending somebody coming up with the materials in which to construct it. Pro-tem, Brevet cards represent a remarkably cheap and robust solution.
Setting out to do a set distance within a certain time limit under certain rules is what an Audax is to me,
This is the bit that I really disagree with (and as an AUK org I hear it a lot).
Audax is about completing a pre-set route (preferably one that takes you far and wide,) to a pre-set schedule (controls and all), not just about riding a set distance. If its just about distance we're back to laps of the park. In fact why bother to go out at all - could do it all on Zwift....
Anecdata: many years ago I was in Milton Keynes and there were signs up to say a long distance running event (I think it was a 24hr event but it was a long time ago...) was being held inside the shoppping mall. That was when MKs indoor mall was an exciting vision of the future...