That was it. I reinstalled Manjaro Plasma with open-source drivers connected to wired networking.
On first boot, I updated all, rebooted, installed kernel 5.13, then rebooted.
On next boot, I did:
sudo pacman -S base-devel git linux513-headers broadcom-wl-dkms
my /etc/NetworkManager/NetworkManager.conf was empty except for comments. All I added was:
[device]
wifi.scan-rand-mac-address=no
I’ve rebooted and checked the logs 3 or so times and the WiFi connects the first try just like Kubuntu.
Thanks, I’ll mark this as solved.