X3 on the above, at least in my world. I don't know how the humidity stacks up between WA and MO, but the local pole barn guys here told me to not completely seal up the shop. It needs some air flow to avoid turning into a swamp with soggy cardboard and rust on everything.
That said, if I had your shop I'd put in a mini-split like this for heat in the winter (I set mine at 50 then turn it up when I go out there) and A/C in the summer. I'd also do a wood stove - there's no substitute for wood heating.
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You can solve your leaky sliding barn door issue as follows. The gap on my door is about 1.5". I made plywood strips about 6" wide and cut to fit the height of the door. It took two 6' pieces for each side of my 12' door. Then I put 3/4" foam pipe insulation on one edge and secured it with a staple gun and then a strip of duct tape on both sides. This is a gasket for the door gap, it takes no time to slip them in when I close the door and they do a credible job of stopping the cold air from coming in to get pushed out of the ridge vent via convection. I use homemade door snakes to close off the floor gap. I'm sure there is a better way to do this but these things are cheap, easy, and work pretty well.
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