Php 7.2+ is great for a lot of saas products and is super fast now. and mysql 8 is rock solid and battle tested in a lot of production systems.

Starting over, I would consider PostgreSQL because it has some nifty features. But now that mysql has added json support, I am less inclined and I see a lot of complaints about performance and scaling that I don't think is as much of an issue with mysql. There isn't a whole lot more out there for relational databases (that doesn't cost a ton). And not using a relational database for most saas systems is just crazy talk.