MacOS – the fonts after macOS 10.13.4 update

Tried clearing the Font caches? Kind of an odd issue- I'm not sure if this is gonna help, but worth a try. Maybe it needs and refresher ;)

defaults -currentHost delete -globalDomain AppleFontSmoothing

sudo atsutil databases -remove and reboot, and maybe it needs and refresher. Its odd since it returns a 3, but maybe something is wrong with your defaults.

Re-pply after rebooting AppleFontSmoothing -int 3 and see if anything first without the string set, you already know that - but let keep the references for others:

defaults -currentHost write -globalDomain AppleFontSmoothing -int 3

I was able to perform the firmware upgrade with no problems. The MacPro upgraded the firmware after I held the power button until it flashed, then prompted me to install High Sierra after. It rebooted a few times in-between.