I'll caveat this before I start - this is my personal opinion in large part from having run the 2017 event, and whilst I was Events Sec at the time the AUK National 400 was resurrected, official Board views may have changed since then.
Recent National 400s seem to have fallen into the bespoke one-off event with fully catered controls. I don't however see that as being the only way, it just the way the event has evolved over the last few years.
The trend for a bespoke event has probably just been down to nothing more than the candidate organisers not having a suitable 400 already on the books. But it should be entirely possible to use an existing event as the starting point. That does however require more than simply just slapping a label on the event - most events on the calendar would need scaling up not just in terms of providing the TLC that's part of the National 400 experience, but also to handle the larger field that goes with it.
Nor do controls have to be exclusively village hall volunteer run style. There are events that use scout groups, WI etc to run controls, not just AUK helpers. And good quality commercial controls should be fine too (provided they can handle the numbers) - think of the likes of the West End cafe and Mariners on the Brevet Cymru. The no-no to avoid is the Ginsters pasty on the 24hr garage forecourt, so a manned village hall control is pretty much mandatory overnight though.
What seems to be apparent from the last few years is that there's plenty of demand from riders (we had 145 on the road in 2017 and including cancellations I had nearly 200 entries, so a 200 rider event should be possible and would be what I'd be aiming for if I were doing it again), but much less interest from organisers. And TBH neither does there really seem that much interest from AUK in really getting behind the event.
When it comes to organisation I wouldn't say it's so much a bigger scale as a different skillset - running this sort of event is more about networking, project management and (most importantly) getting the right people involved rather than the "do it all yourself" approach.