Sweet - thanks so much @daniel and @aparcar! The whole dynamic sysupgrade/auc process fantastic, and it's terrific to be able to leverage it for these release candidates too.
Just upgraded all my x3 RT3200's connected via WDS (each with their own package mix) and so far so good.
FWIW I have been using this setup for business and personal use based on the master snapshots for months without issue, whilst waiting for (more) stable release(s). I use: a) WireGuard; b) adaptive bandwidth CAKE on LTE connection notwithstanding VPN PBR; c) WDS extended WiFi and separate WDS extended Guest-WiFi, both with fast transition to support roaming. And it all just works beautifully. So a huge thanks to the OpenWrt devs for making experiences like mine possible.