How to override existing Documents module to use the new S3 Storage option in v8.9?

I’m installing a new (and first) Suite CRM system for my company and it’s all gone OK so far apart from getting the new S3 storage configuration introduced in 8.9.0 working with the pre-existing Documents module as that still saves them to the legacy /public/legacy/upload.

I’ve followed the route of adding a vardefs override in public/legacy/custom/Extension/modules/Documents/Ext/Vardefs/ and called it
_override_documentstorage.php

Using File Field Configuration :: SuiteCRM Documentation as a guide, it’s contents are as follows

After clearing the caching and using the quick repair and rebuild option in the admin UI, I can see that it has been merged into public/legacy/cache/modules/Documents/Documentvardefs.php

image

But it still just saves it in the same legacy location meaning that it hasn’t even tried to use the AWS storage, let alone errored. This is further evidenced because the local fallback path for private documents as defined in config/packages/flysystem.php is

image

Not the legacy path of old.

So, the question is what, if anything, have I missed?