Has anyone successfully set up Collabora office with Caddy?

Yes Yes I did, sorry! Collabora CODE server configuration in Caddy.

office.DOMAIN {
        @collabora {
                path /browser
                path /browser/*
                path /hosting/discovery
                path /hosting/capabilities
                path /loleaflet/*
                path /lool/*
                path /cool/*
        }
        @local-ip {
                remote_ip private_ranges
        }
        reverse_proxy @collabora https://nextcloud-office:9980/ {
                transport http {
                        tls_insecure_skip_verify
                }
        }
        reverse_proxy @local-ip nextcloud-office:9980 {
                transport http {
                        tls_insecure_skip_verify
                }
        }
}

Nextcloud section, may look different from yours I use the FPM image but it should be similar, enough. There are also some limitations on the urls based on local vs public access but that is an attempt at hardening rather than necessary configurations.

nextcloud.DOMAIN {
        root * /var/www/html

        file_server

        php_fastcgi nextcloud:9000

        header {
                Strict-Transport-Security max-age=31536000;
        }

        @phpFiles {
                path_regexp phpfile ^/(remote|public|cron|core/ajax/update|status|ocs/v1|ocs/v2)\.php
        }

        rewrite @phpFiles {http.regexp.phpfile.0}

        redir /.well-known/carddav /remote.php/dav 301
        redir /.well-known/caldav /remote.php/dav 301

        @local-ip {
                not remote_ip private_ranges
                path /settings/admin
                path /settings/admin/*
                path /settings/users
                path /settings/users/*
                path /settings/apps
                path /settings/apps/*
        }
        #respond @local-ip 404

        @forbidden {
                path /.htaccess
                path /data/*
                path /config/*
                path /db_structure
                path /.xml
                path /README
                path /3rdparty/*
                path /lib/*
                path /templates/*
                path /occ
                path /console.php
        }
        respond @forbidden 404
}

Then in nextcloud you just point it to the CODE server domain above & you even have tls securing the communication layer.