I haven't read through the entire thread, but these chickens are in Aus, right? We don't have as wide a variety of breeds as other places so if it doesn't match a breed we have than it's likely a cross. Any breed rare enough that's barely talked about here I
highly doubt would end up in the hands of a rescue.
Some bantam breeds I can remember are Sebrights, Japanese bantams, Modern game, Dutch Bantam and OEGBs. They're not any of the first four breeds. I don't know enough about Dutch bantams and OEGBs to say, but I think you've been given some good replies.
I doubt we're lacking either. It's entirely possible you don't see many crossbreeds advertised because people are more interested in purebreds and mixes are likely sold more by word of mouth.
I've got two mixed hens, one from a friend and one I bred. I have one mixed cockerel and a heap more due to hatch

. I've also got a few "purebreds" who I suspect of having been crossed at some point.
I've even bought mixed breed hatching eggs from a rare breed breeder