Welcome @lebaux 
If you’d like to opt for Adminer instead of phpMyAdmin, it’s enough just to uninstall it and disable respective nginx config files in “/etc/nginx/conf.d/”, eg. by appending “.disabled”.
Recently, there was needed customization per each client, hence I chose to create “admin” subdomain per each client’s domain using Hestia CP.
If you rather prefer changing nginx config templates or Hestia system ones, you can take a peek at the [How-to] RainLoop in Hestia CP tutorial how to achieve this…