There is a brand that is sold on
Baum-shop24.de or Baum BMW here in Germany called ELRING. These guys vouch for it.
Stick with OE or at least ELRING and don't deviate. I can't believe how that gasket failed to like that. seems burned.
Going forward, you do not have to do an intense flush of the coolant circuit. Just used filter water and detergent and run an air purge procedure and keep flushing it until it becomes a decent in color. Once you are happy, use G48 or better specd coolant to suspend minerals that the filtered water may have left behind. Flush about a week later and then maybe 2 weeks from that until most of the oil is out.
I would definitely replace the expansion tank first and go from there.