Great write-up! One question I had was around the use of keepalives. There's no mention in the article of whether keepalives were used between the client and reverse proxy, and no mention of whether it was used between the reverse proxy and backend.

I know Nginx doesn't use keepalives to backends by default (and I see it wasn't setup in the optimised Nginx proxy config), but it looks like Caddy does have keepalives enabled by default.

Perhaps that could explain the delta in failure rates, at least for one case?