Mangling non-main-entry-export names

Suppose I have the following two files:
_prepare.mjs
```
const _stringifyCallback = (_, value) => {
if (!value.isSJS) return value;
return `${value.type}__sjs`;
};

const _prepare = (schema) => {
const _preparedString = JSON.stringify(schema, _stringifyCallback);
const _preparedSchema = JSON.parse(_preparedString);

return {
_preparedString,
_preparedSchema,
};
};

export { _prepare };
```

and

main.mjs, which is the entry point.
```
import { _prepare } from './_prepare.mjs'

function a() { _prepare({}); }
function b() { /* do other stuff */ }

export { a, b }
```

This results in the following minification:
```
const _stringifyCallback=(e,r)=>r.t?`${r.type}__sjs`:r,_prepare=e=>{const r=JSON.stringify(e,_stringifyCallback);return{_:r,p:JSON.parse(r)}};function a(){_prepare({})};function b(){};export{a,b};
```

However, when monitoring closely `_stringifyCallback` could be mangled, but is not, and also `_prepare` could be mangled, since it is not exported from the main entry point.
Is there a way to achieve at least one of the mangling?

The following mangle.json did not change anything (I tested a lot):
```
{
"minify": {
"mangle": {
"toplevel": true,
"properties": true,
"keep_fnames": false
},
"compress": {
"keep_fnames": false,
"reduce_funcs": false,
"passes": 2
},
"toplevel": true
}
}
```