I've experienced what you describe as well. No two digital or liquid thermometers reading alike and varying widely from one another. I've ordered mercury thermometers which are considered to be the most accurate of the liquid variety to add to the mix. The mercury's are not easy to find, but I found one sold to veterinarians and the photo industry uses them too. There's a lag effect with the digital thermometers that has to considered. I tested all my thermometers (13 of them and counting) first by placing them on a hard surface in a small room where it was easier to maintain a constant higher temperature than the rest of the house. Then I used a good quality infrared/laser gun to read the surface temperature for comparison to each thermometer. I ignored the outlier readings and averaged the readings that were close to one another in order to arrive at a theoretical mean number that I could compare each of the units too. Then I placed them in my incubator(s) in various locations and noted the readings there as well. When my mercury thermometers arrive (accurate medical thermometers tend to be in short supply right now), I'll run the tests again.
Some recommend immersing liquid thermometers in ice water to verify that they read 32 degrees. I've done this to prove my liquid thermometers were reading correctly at 32 F, but found them to vary when the temperature was raised to 100 degrees even with the same manufacturer.
One can calibrate for humidity by placing a digital unit inside a kit sold to cigar buffs that sells for around 10 bucks on Amazon. There are kits for high and low humidity calibration tests available for cigar buffs or one can just Google for the salts that are needed to put into a sealed bag along with the hydrometer for 24 hours to verify if it's reading correctly or not.