With your fluctuating temperature readings and resulting issues at hatch I would consider getting a separate temp/hygrometer, calibrate it, and then see what your incubator is actually running for temp/humidity. Govees are highly recommended around BYC.
Like said, it does sound like your temp was running slightly low for the late hatch. My temp in mine seems to run slightly high as every hatch I've had has been day 20.
And I'll throw this little tid bit into the mix. If your ordering BBS Copper hatching eggs I would consider doing a Marans eggs only hatch and ask the breeder for suggestions on humidity.
The reason I say this, is I find a lot of Marans breeders I've talked with suggest running a lower humidity than normal eggs (what's called a 'dry hatch'), mid to upper 20s and not over 50% at lockdown.
The thicker shells and darker pigment of the eggs retain more moisture requiring less humidity during hatch.
More thoughts to stress over...