I have zero knowledge on getting Apple branded hardware working with LInux and even less with the broadcom wireless so what I can only offer my thoughts.
I suppose you are familiar with the Arch wiki on the subject?
If it is a kernel regression - I suppose the system is using stable branch - it may be worth testing on unstable branch - which is not that unstable - as it comes mostly from Arch - and kernel rebuilds happen more often there - and as such kernel regressions are fixed a lot sooner.
But that only points to something you have tried.
Perhaps you can clean up the testing - remove all broadcom packages - recheck the modules-load.d and modprobe.d for possible blacklisting - then reboot and try -broadcom-wl-dkms.