Not to be mean, but from my reading this needs Heroku free tier which no longer exists? I am very sorry if I got that wrong.
constexpr variables are constant and usable in constant expressions
constinit variables are not constant and cannot be used in constant expressions
constexpr can be applied on local automatic variables; this is not possible with constinit, which can only work on static or thread_local objects
you can have a constexpr function, but it’s not possible to have a constinit function declaration