The most battletested way is via bindings to other languages. Ex. mlua with luau sandboxing Or Javascript via v8 or deno_core

There are also a few languages implemented in rust like rhai

There is also the option of compiling the user code to wasm and using wasmtime to run it

Custom license that doesn’t meet the FSF’s definition. Tldr restrictions on redistribution and minor restrictions on modification. It isn’t on fdroid’s main, but they host a fdroid compatible one with a out of date version of Grayjay

I’d look into the git-maintenance’s prefetch task. From what I understand, that is more or less what you are looking for. Then just run any old http(s) server and clone them from that https://git-scm.com/docs/git-maintenance