I wouldn't put effort into changing if you have no troubles with nginx. Caddy has some neat features like live-configuration and built in LetsEncrypt. Plus the JSON configuration is a great idea, once I get it working.

The real reason is the thing you highlighted: "I can see there are some NGINX Plus features that are free in Caddy." Sadly it seems that NGINX is now crippleware. Personally I find it risky to depend on open source organizations who refuse to accept important features to the project, so they can sell those features as proprietary.