Good to know, however "when the keys are know" refers to a second visit (or request) of the site right ? That isn’t helpful for the first data paquets - at least that what I understand from the site.
Without cached data from a previous visit, 1-RTT mode works even if you've never vistited the site before (https://blog.cloudflare.com/rfc-8446-aka-tls-1-3/#1-rtt-mode). It can fall back to 2-RTT if something funky happens, but that shouldn't happen in most cases.
0-RTT works after the first handshake, but enabling it allows for some forms of replay attacks so that may not be something you want to use for anything hosting an API unless you've designed your API around it.
