It's safe to say that this idea has been validated now. We have 1000 customers and $42k MRR. When we started we wanted to be the best website builder for churches. We used WordPress to get up and running quickly and put a ton of effort into custom functionality. Basically, you can't install a plugin and rebuild what we have currently done.
I went down the route of building a headless option for WordPress using React for a custom Admin Interface. It worked great but at the end of the day, it was still hacking WordPress to get it to do what we wanted.
A month ago, I decided to start fresh and give myself two weeks to build something custom. I focused on Authentication, Stripe Subscriptions, and a simple UI. It worked great! So well, that I can't imagine not using this moving forward.
Today, I forked my repo and started Version 2 of Version 2.
Wish me luck!