Hi, I have Wren(not 2).
I don't recommend Wren to you, because I think there are not so much differences between Chinese one; hard to make sound, easy to be broken.
Have you ever disassembled your concertina? Cheap concertina has same mechanism and it is "completely different" from above mid-range model.
Mechanism of a chap model
Mechanism of a mid-range model
I think Swan is good choise, I have not played it yet but I heard from my good friend that Swan has nice cost performance.