So before doing anything, make sure that inside the configs for both backend and proxy, the mysql credentials are correctly set. By default there's a 2 second delay between player removal/add.
The database type doesn't necessarily need to be mysql, you can leave it as it is normally. Make sure though that this is set to MySQL:
# ######################################################
# PROXY #
# ######################################################
proxy:
# Master switch for proxy functionality
enabled: false
# The name of this server as registered in the proxy network
# This should match the server name in your proxy's configuration
server_name: 'dev2'
# Data storage method for cross-server communication
# Available options: Redis, Lettuce, MySQL
# Note: If using MySQL, ensure correct database details are provided in the "database" section
storage: "mysql" # <-- !!!!
# Redis connection settings (RECOMMENDED)
# Only relevant if storage matches set to "redis" or "lettuce"
redis:
# Redis connection URI
# Format: redis://[:password@]host[:port][/database]
uri: "redis://:password@host:port/0"
Read more about the other settings inside the config itself.