Sorry, no kub expert here. I'm not sure how to teardown just the database, but I assume you could write a script to connect to the backing database and drop the tables/schema, etc. That might be quicker than dropping the entire cluster.
You can use the database.maximum-pool-size value in the fusionauth.properties file prior to 1.19.x ( it was just not documented) , but if you want to use the environment variable version is not available until 1.19.x.
It is worth noting that MySQL group replication will not work. It requires a primary key on each table. We do not meet this requirement and have no plans to adjust the schema for this style of replication to work. Other types of replication that do not require a PK on every table should work.
The particulars of DB clustering are outside of the scope of what we can assist with, however.