I switched to booting from an SSD for my Raspberry Pi 4B I had been evaluating as a desktop replacement. In the process I also switched to Raspberry Pi OS 64-bit beta (you can get the Lite version here; there’s a Desktop version too).
I had to install Raspberry Pi OS to a micro SD card, flash the firmware to the latest version, and then follow the steps in this article to get things working. AFAIK Manjaro too should have let me update the firmware but I couldn’t get it to update (it didn’t find any new firmware and I am sure it’s an oversight on my part than any limitation of Manjaro – I simply didn’t spend too much time Googling on it). Since I was going to use it headless anyways, I didn’t have need for Manjaro any more and decided it’s better to stick with the official OS. One nice thing about using Raspberry Pi OS is that you get instructions for it easily. (The 64-bit version seems to be different from the 32-bit version in that at least all the repos point to Debian than Raspbian so I wonder if there are other differences).
It’s important to update the firmware and then use
raspi-config to change the boot order to USB. I was initially following a different set of instructions and that didn’t have this step (as a result of which USB boot didn’t work for me – the Pi kept complaining that it couldn’t find the microSD card and gave up).