You need to update the firmware using an internal apple SSD. Don't think there is any other way to do it. Maybe the apple store can help you update the firmware using their own ssd?
I can't format the only ssd that I have, I just can't, it's from my work pc, I just can't do it. Apple Store won't help me, I already asked, they can't help me. If I install the MacOS to the external ssd, won't it update the firmware?