The bren mag feeds more reliably than the stanag, and you retain lrbho and magazine release. As far as differences between stanag and bren, it would be the fastest to prototype with some dimensional changes. You can mill an existing XCR lower to fit them. We could probably design a steel lower insert that allows you to swap between bren (no insert) and stanag.
I agree that availability is an issue, but it's the cheapest shortcut in development time and prototyping vs the other options.