I'm not sure I understand what ProxySQL is...
What's wrong with master/slave (or read/write replicas) functionality already built into database servers? Why does ProxySQL need to go on-top?
If you need this type of set-up, you probably don't use ECP (sorry, just my two-cents). You'll end up building infrastructure from the ground-up and/or using for example DigitalOcean's Database product which includes scaling, clustering, replication, and automatic failover.
I tend to agree with necrom. There's a reason why GoDaddy has multiple database servers and it's not because of geographical location. If you have 1,000 clients and one database goes down, even with automatic failure, you end up having downtime for 1,000 clients. 10% (100) clients going offline is far better than 100%.