What matters with a startup is executing, and that means using tools that let you execute well. If you're most familiar with PHP and MySQL, then that's what you should use.
As someone biased against your tech stack, I fully support your decision and think the expert and the investor have no idea what really matters, hence I stand by that they're a bad investor.
On the other side I'm evaluating CL for my next endeavor since I personally find it to be my most productive language, but realistically I'll settle for Clojure and even then I'm worried if that is a bridge too far when it comes to the whole funding/due diligence issues.
I'd love to know what their export advisor considers the right decision?
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.
But I assume you have a relational database somewhere to manage users and do other relational-type things?
They pretty much ghosted us soon after that meeting. Though they did not specify that as the reason, it seems most likely that their advisor told them we don't know what we are doing technically.