Problem #1 is getting sensor date into Hubitat. If the sensors you have or are assuming you'll get aren't able to inform Hubitat, you won't get past the starting line.
Within that, you say "get a constant read out on temperature and relative humidity" and the word constant is a recipe for overloading the Hub. I doubt the greenhouse needs sub-second response. My guess is that if it adjusted once every 5 mins, it would be pretty amazing.
You may need to create a "Converter" to take your desired sensors and convert them into data for Hubitat. Search here for Hubduino as that's a tiny computer that will take non-compatible sensor data and provide it to Hubitat.
Problem #2 is getting Hubitat commands out to the actuators. It's really identical to problem #1 except the direction of data. If your actuators aren't compatible, you need to convert them to be. Again, the Hubduino project could work as well.
You haven't named your sensors and actuators and I don't think I've ever seen a PSGH. 
Once the data is being fed into Hubitat, most of the rest of the problems are doable as you say: "this shouldn't be too much for the Hubitat to tackle. " Once you have the means to see the data (your sensors) and to move your actuators, the remaining is all the fun stuff of detecting nuances in the data and reacting to it.
Fun project... keep us appraised. 
