Well that turned out to be a very simple fix once I understood how the path matching has changed.
All that was needed was to remove the root / from the reverse proxy line or change it to /*. A small detail I didn’t pick up on initially. It explains why it only loaded the root path stuff and nothing else.
@matt after a few “getting hit on the head lessons” I’m finally feeling some love from V2. I have yet to xfer all the rest of my V1 caddyfile so I expect some more issues (e.g. nextcloud, quasar webapp) but hopefully won’t be too painful from here on out. So yes thx for your, @Whitestrake, @francislavoie and other’s efforts. If you wanted V2 to be a giant killer (nginx,apache) you have succeeded and the web management interface would be the dancing on their graves.