What if the message is a configuration message that can’t be split?

And how would it be split?

Or should there be a mechanism for reassembling message frames?

That's what I mean. Like SMS it's a syntax that would allow for smart clients to re-assemble the message but is visible in plain text for backwards compatibility

Why is the config message bigger than the standard minimum?

Because a message can contain message tags, and other config stuff.

Imagine you have a 513 byte tag in a message.