thanks,
you remind me i can customize the stdout, stderr path.
just send a pr, before pr merged,
here is my solution:
you can just go to brew --cellar caddy folder and edit homebrew.mxcl.caddy.plist and add
<key>StandardOutPath</key>
<string>/usr/local/var/log/caddy.log</string>
<key>StandardErrorPath</key>
<string>/usr/local/var/log/caddy.log</string>
after RunAtLoad.
then brew services restart caddy
tail -f /usr/local/var/log/caddy.log