Caddy is a bit quick and dirty, rapidly-developing, with neat plugins but hard to configure for more complex scenarios and too light on the docs (IMO).
HA Proxy is robust, comprehensive, mature, and bulletproof. It's basically boring because it works so well.
If you have to choose only one to learn, choose HA Proxy.
I wanted to try it out just now but hit a roadblock immediately - it cannot automatically obtain and maintain TLS certificates. You have to use an external client (e.g. acme.sh), set up a cron to check/renew them, and poke HAProxy to reload them if necessary. I'm way past doing this in 2023.
If getting Let's Encrypt to work with HAProxy is your only struggle, you'll soon overcome it and be loving HAProxy. And there are multiple ways to set up Let's Encrypt, if you don't want to use acme.sh. For example, you could use certbot. There are blog posts that cover that pretty well.
