Surely "read object" and "write object" are not hard to migrate to local file system. You can also use Apache OpenDAL which provide the same interface to both.

Yeah, unless you have the raw S3 API throughout your codebase you should be able to write a couple dozen lines of code (maximum) to introduce a shim that's trivial to replace with local file access. In fact, I've done this in most projects that work with S3 or similar APIs so I can test them locally without needing real S3!