I think that OP really meant that in his/her mental model of things, answers already known and battle tested for numerous cases [by using Nginx] - making HA wit

I think that OP really meant that in his/her mental model of things, answers already known and battle tested for numerous cases [by using Nginx] - making HA with keepalived? Check. Making logging to be buffered to save iops? Check. Implementing ratelimits and custom logic? Check. Keepalived is not really bound to Nginx by any means and should perfectly fine work with Caddy too.