Even simpler than that, is there any reason not to go with something like a Vanguard Lifestrategy which implements something quite close to a Bogleheads mix? Asking sincerely as that’s what I’m using now, and it’s just so easy it feels like there must be some unanticipated downside.

Those funds have higher costs. If you want to do that (and it's a good idea) look at the portfolio allocation and then buy the ETF version of the underlying assets.

This is really the only downside (beyond them being perhaps too conservative - but you can adjust by selecting a different “year”).

I agree. Buy some low cost diversified funds through Vanguard. I cannot understand why the average investor needs anyone else in the loop.