I love hearing about developer experiences by solo devs. As a potential solo dev, I was actually looking to go the Spring Boot route. I think Java has gone in a better direction the past few years, and I prefer it over PHP due to static typing, functional programming (to a point), and the tooling. Spring Boot seems to provide a lot of the same development productivity. My only other experience is with Rails and Django, although those have been with small projects.
I always find these technical decisions to be difficult to gauge when you're starting off.