But for the sake of readability it would be much better to simply have username@mysite.com than username@mail.mysite.com

That's kind of unrelated. You can configure a mail server at mail.mysite.com to handle mail to/from username@mysite.com. You don't need a proxy for that.

But what if I do want my services to be accessed through mysite.com directly instead of a specific per-service subdomain?

If they're all http(s) services, then that should be possible. I don't know anything about caddy, but with apache or nginx you can proxy based on path, so I'd assume you can with caddy also.

For example mysite.com/chat could route to your chat app, mysite.com/webmail route to your webmail app, etc. But this isn't necessarily plug-and-play, because depending on the app you might need to set up proxy rules for cookie rewriting, link rewriting, etc.

If you want to proxy non-http(s) traffic from 1 port to multiple destination apps, then it gets a LOT more complicated.

You don't ssh in at ssh.mysite.com. Port 443 is form HTTPS traffic only. SMTP and IMAP traffic will require additional ports exposed that don't need to go through caddy. Only the web UIs for your email will be exposed on the subdomains via caddy.

Registering a mail domain is done through DNS records too not via caddy.

So, you want to have two services accessible via the same URL? How do you want that to work? Can you give a list of services and the URLs you would like to access them by?

At least for mail, its actually not too hard, because SMTP/POP etc are all on different ports, so you can host your mail at mysite.com, and still have a website at the same address.

It can be done, but only if the service your trying to use makes all of its requests using a specific port. There's no way to do it with a reverse proxy to my knowledge though. I've got both my email and my blog at myname.tld, for example. I just had to configure both of them with DNS.

If what your trying to do is have all requests for one port be directed to another port... your out of luck. I've been trying for ages to have nginx forward all 443 traffic on proxy.mynane.tld to 30018 and 80 traffic on proxy.myname.tld to 30024 but no luck so far.

Is that how you think the Internet works?

what if I do want my services to be accessed through mysite.com directly instead of a specific per-service subdomain

Nope, not how the Internet works.