I have my issues with coinbase (like them randomly deciding to declare my purchases high-risk after 4 days of entering said purchase and making me miss out on HUGE profits AND after I've successfully bought through them for thousands of USD for months) but it sounds like you're just a mad bro.
Strange, I've heard about this "too high-risk" claim several times recently, it seems that they are a bucket shop that bet against customers
I am not against bucket shops, I have several accounts at bucket shops, but even bucket shops have their business model, if a bucket shop constantly cancel the order, means they are just too illiquid to be fully operational
Anyway, a bucket shop is not an exchange, but a casino. Investing 5 million in a bucket shop is not some thing worth mention