Beautiful greenhouse! Wow! A tube of indoor/outdoor caulk and some scraps of the twin-wall poly will take care of all those small gaps.
For heat, based on my experience I’d stay far away from electronics. The heat and humidity in a greenhouse are hell on electronics. I’ve had way too many gizmos fail and I’m back to solid state especially for heat. If you lose your heat mid-winter you can lose everything in your greenhouse overnight. There are a million ways to do it using electronics but there are easier ways.
The easiest thing to do is buy a Thermacube. Just plug it in, plug a space heater into it and you are done. If you want more control get a Peco. They work for either heating or cooling (buy two if you want to do both heating and cooling) and mine has been foolproof. To switch between heating or cooling you are wiring it in two different ways. It is very simple if you have any basic knowledge of wiring. I run these to an outlet then plug my heaters into that outlet.