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Erlang also has reference counters for things like strings that are immutable and can be shared between threads (processes in Erlang).

Overall this is a good model. Use GC for small per green thread heaps. Then use reference counters for shared immutable structures that cannot form cycles and copy everything else.

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Erlang only uses reference counting for binaries larger than 64 bytes, everything else is allocated on the process heap (or in heap fragments) and copied. Just that is enough to have a beneficial effect though, since large binaries are relatively common in practice, and are frequently passed around from process-to-process.

And if you have any other global state you want to pass around, you can pull off a clever trick by passing it around as a binary and then unpacking it as needed within caller processes.

AFAIK this trick is why BEAM files use an IFF-derived format (easy to parse individual chunks out at runtime), and why erlang:module_info/{1,2} are the way they are: working with module metadata literally just means asking the code-server process for the (shared refcounted) module binary, and then parsing it yourself.

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