It’s not a work-around, that’s the standard way of installing Magisk.
If it were possible to get root just by installing an app, that would have quite some security implications.
for fastboot boot to work, a bootable slot must be active.
In your case no slot is selected and b is marked as unbootable, so you should activate slot a:
fastboot --set-active=aThese ZIPs are meant to be installed using TWRP, not fastboot.
You could install a ROM using fastboot, which would probably also fix your phone:
https://support.fairphone.com/hc/en-us/articles/360048148532
https://doc.e.foundation/devices/FP3/install