Strangely enough, in this particular context, I'd say PHP is better than Rust, cause at least you get a garbage collector.
Solid logic with sticking with what you can prototype in quickly. That's a lot more important than performance. However, people hate PHP, so it's gonna be hard to find help (I assume it doesn't matter, it seems like a solo mission :)
Perhaps, this type of discussion should be avoided. Maybe some kinda FAQ section where you explain why Lemmy isn't satisfactory for you.