The complexity you're hitting here is the main reason I'm a fan of creating a dedicated library for dealing with these problems, rather than trying to handle them directly on the builtins.

Given a bufferlib module, for example, you could have a higher level API like:

    def snapshot(source, *, result_type=bytes):
        ...

That handled any source object that supported the buffer protocol, and any target type that accepted a memoryview instance as input to the constructor.

    # Default bytes snapshot
    data = snapshot(original)

    # Mutable snapshot without copying and promptly releasing the view
    data = snapshot(original, result_type=bytearray)

The start/stop/step or length+offset question could be handled at that level by allowing both, but also offering lower level APIs with less argument processing overhead:

    def snapshot_slice(source, start, stop, step=1, *, result_type=bytes):
        ...

    def snapshot_at(source, *, offset=0, count=None, result_type=bytes):
        ...