I found my last attempt here (before I gave up). I also spent a lot of time getting the basic logs working, by installing modules that wasn't part of the standard distribution

    {
      "logging": {
        "logs": {
          "default": {
            "level": "INFO",
            "encoder": {
              "format": "transform",
              "template":"{common_log}"
            },
            "writer": {
              "output": "file",
              "filename": "/var/log/caddy/access.log",
              "roll": true,
              "roll_size_mb": 5,
              "roll_gzip": true,
              "roll_local_time": true,
              "roll_keep": 5,
              "roll_keep_days": 7
            }
          },
          "dev_access": {
            "level": "INFO",
            "encoder": {
              "format": "transform",
              "template":"{common_log}"
            },
            "writer": {
              "output": "file",
              "filename": "/var/log/caddy/dev_access.log",
              "roll": true,
              "roll_size_mb": 5,
              "roll_gzip": true,
              "roll_local_time": true,
              "roll_keep": 5,
              "roll_keep_days": 7
            }
          },
          "errors": {
            "level": "ERROR",
            "writer": {
              "output": "file",
              "filename": "/var/log/caddy/error.log"
            }
          }
        }
      },
      "apps": {
        "http": {
          "servers": {
            "srv0": {
              "listen": [":443"],
              "logs": {
                "default_logger_name": "dev_access"
              },
              "routes": [
                {
                  "match": [
                    {
                      "host": ["<redacted>"],
                      "path": ["/backend/*"]
                    }
                  ],
                  "handle": [
                    {
                      "handler": "subroute",
                      "routes": [
                        {
                          "handle": [
                            {
                              "handler": "rewrite",
                              "strip_path_prefix": "/backend"
                            }
                          ]
                        },
                        {
                          "handle": [
                            {
                              "handler": "reverse_proxy",
                              "upstreams": [
                                {
                                  "dial": "localhost:8080"
                                }
                              ]
                            }
                          ]
                        }
                      ]
                    }
                  ]
                },
                {
                  "match": [
                    {
                      "host": ["<redacted>"]
                    },
                    {
                      "file": {
                        "try_files": ["{path}", "/index.html"]
                      }
                    }
                  ],
                  "handle": [
                    {
                      "handler": "file_server",
                      "pass_thru": true,
                      "root": "/home/server/web/dist"
                    }
                  ]
                },
                {
                  "match": [
                    {
                      "host": ["<redacted>"]
                    }
                  ],
                  "handle": [
                    {
                      "handler": "rewrite",
                      "uri": "/index.html"
                    }
                  ]
                }
              ]
            }
          }
        }
      }
    }