SAPs can suck a nut.
I'd look into a remflex gasket for the exhaust, anything post turbo, they're my go-to gasket choice. Maybe a slight hassle for something "custom" like your 3" to 2.5" fitment, but once I poked around on their site and found the right gasket (and sometimes modified the bolt pattern slightly...) I never had one leak or fail. Not re-usable and a bit pricey, but better than slathering one up with RTV or stacking gaskets and hoping they seal.
Two bolt flanges can suck a nut too
BMW is looking good!