Everyone has there own ideas of monitoring data so I will mention mine and some of the methods I use etc.
Regarding your twin walled polytunnel, like the idea of using air, but from my small use of double layer bubble wrap, wonder how long your “seals” will last in the hot uv light and cold freezing nights ? and will you end up having to keep a pump running 24/7 as the leaks grow ?
Don’t know what air gap you are using but I used some 1" cubes of polystrene every foot of so to keep the two layers apart, expect there are some ready made such spacers for polytunnels, even with leaking seals it should still maintain the air gap.
Hydrometer, assume you mean a soil moisture gauge ? - no never used them as think the cheap stuff is no much use and the rest are rather expensive, though only from what I read.
Would think for a polytunnel ( how large ?) something like a timer /solenoid/seep hose might be more suitable, in warm conditions and open drainage it will not be easy to over water short term.
Readings, I know some folk want to sit and watch every temperature change throughout the day, but I find a simple daily min/max reading is all that needed.
Your automation, be it auto roof vents or cooling fans / or heaters in winter should take care of anything inbetween, though is max temperature that will be your main concern.
( can I ask what country you are in ?)
For controlling am sure an Arduino will do, while I would suggest a simple Uno for developing each module on, you might have to go to an Ardunio Mega 2560 for the complete project.
Regaring wifi and all that, seems a bit old fashioned to me, I made an automatic SMS controller that will send hourly ( or whatever) status and alarm reports to your phone, including a small battery backup so you still get notification when the mains goes off.
Its all there for you to make your own custom controller, just needs putting togther, if you have the time and inclination, as my old schoolteacher used to say ;D
Plenty on help in the forum as you can see.