Just in case you didn’t see this in other posts: If you want to keep FPOS for now, it might be less risky to avoid flashing the boot.img patched by Magisk to the phone.
You can instead have fastboot boot from a patched image located on your computer. That way you can leave the phone partitions unmodified and still get root access temporarily until you reboot. I do this to make my banking app happy and I only need root for backups or rare one-time changes anyway. The banking apps I know don’t need locked bootloaders, but they do detect Magisk even with hide enabled.
I recently used this approach to port my FP2 apps to my new FP4 with Titanium backup and it worked very well. It all depends on how often you actually need root and whether you are happy with FPOS.
I can confirm that this is the case. Booting continues after a few seconds, but you can press a button to pause on the screen.