We use proxysql-admin in the operator to configure proxysql.
So whatever changes you make in the hostgroups - they will be overwritten by proxysql-admin and the operator.
We have this issue right now that is exactly about using only one server for reads: [K8SPXC-735] ProxySQL uses only one server for reads - Percona JIRA
We are working on its resolution, but it requires some work.
As a workaround some of our users tend to use haproxy with Operator and setup proxySQL as a standalone server.