Perhaps the need for the second boot before pamac-updater runs properly, is because of WiFi. My guess is that the (pamac) updater would work immediately after first boot, IF the computer is plugged directly into the internet via Ethernet cable. Because any pamac/updater code will see the internet immediately, even if it is a behind-the-scenes updater daemon or module running during bootup.
But if the internet connection is by WiFi, well, after the first bootup, some updater code is running but you have not set the WiFi password yet. So the updater business thinks it can’t reach the worldwide software database. So after the first boot, set up the WiFi, and just reboot. From then on your Linux computer will “see” the internet much earlier in the booting process, and by the time the boot process settles down the pamac-updater will run OK.
So overall we can just observer that if you are using WiFi, don’t expect pamac-updater to work right until you have set up the WiFi and then done one more reboot.