Congrats, it looks great! How did you get the testimonials? Reach out to "power users" (based on usages numbers)? Or did you specifically recruit artists to your platform?
Now I email everyone who takes a paid plan.
But yeah to answer your questions I just email everyone like a human being. Not like some marketer gasbag. I build relationships, make sure I'm on point when someone has an issue, and then just ask them for a testimonial.
All the testimonials on there just are are legit paying customers.
Customers really appreciate it. I guess its not for everyone, and it's not infinitely scaleable but it is highly valuable in the early stages.
If you feel that a real human person is behind all that, and not a faceless company, you're also much more likely to share feedback. (Nobody would share improvement ideas for Windows with Microsoft, since you can be sure that those go straigth to /dev/null (or, more appropriately, the recycle bin).)
I get so much response when I email people. Every email is hand typed and I just say whatever comes to mind at the time.
Its super helpful for getting feedback and getting to know my customers.