Garbage collection becomes almost a necessity when you have enormous concurrency and parallelism. I wonder whether you have experience writing such programs with pthread and malloc/free?
Go, on the other hand, does have scaling issues. So do most GC'd languages in fact, as the GCs want to periodically park all threads to walk their stacks. Even the latest & greatest GCs, like Shenandoah, still gotta park all them threads periodically for milliseconds at a time.
No, you can just use Rust, which provides thread safety without garbage collection.
C++ has libraries threadsafe shared pointers, interthread message passing, transactional memory, futures/promises, work stealing queues, and access to all the OS primitives to build anything else.