Hi 
Have a look at Nginx return directive in caddy -- return json - #3 by IndeedNotJames.
You technically don’t need the dougfredericks.space:8448 if you delegate via .well-known/matrix/server to { "m.server": "matrix.dougfredericks.space:443" }.
You would need to customize the handle_path from the linked post and add something like
dougfredericks.space {
handle_path /.well-known/matrix/* {
<all the other things>
}
}
to your Caddyfile and you should be all set!
Hope that helps, but feel free to ask more questions 