I wouldn't post this but for the worthiness of the source. I can't speak to it personally but if someone hits on it while searching for this solution then good.
The write up is accurate with regards to some information, but leaves major gaps in others. Without the adapter model number the entire article is somewhat irrelevant. All Broadcom adapters are not the same and his failure to recognize this fact makes his solution inappropriate in many cases.
The b43 kernel module he recommends only works better than the wl driver in a minority of Broadcom adapter cases. There are quite a few Broadcom models that the b43 kernel module will not work with at all. The driver option that he recommends also only improves the signal in a minority of adapters. Again the adapter model is the determining factor as to what solution needs to be applied.
The advice given in the article in my opinion while not bad advice per se, is not applicable to many Broadcom adapters.
Thanks for this but I wonder how the live ISO can work perfectly, yet the installed one not. Is it that the installation process layers a non-working driver / kernel module atop a working one?
This happens in many cases because the live installer uses the b43 kernel module, but the final installed version uses the more widely recommended broadcom-wl driver.
His model of Broadcom adapter is obviously one of the models that does not work well with the wl driver that is installed by default. Generally most (but not all) broadcom adapters perform better with the wl driver.
The kernel version can also have a great bearing on how many Broadcom adapters perform.
He also mistakenly states that the wl driver is a free driver (it is not). He also must not have uninstalled the wl driver correctly through Manjaro Settings Manager. Otherwise, he should not have had leftover blacklist files on the kernel modules that required manual removal.
As I stated the advice is not wrong, but is not applicable to the majority of Broadcom adapters.
FWIW my 11 year old Dell with a BCM4312 card performs much better after building the b43-firmware from the AUR and removing the wl driver. Signal strength is better and signal monitoring via conky is available.