Welcome, Kathie!
It is a friendly bunch here. Kathie has been casting around a little trying to figure out where she would fit in to a gardening forum. She was thinking that her 2,000 foot elevation didn't exactly qualify her as a mountain gardener and her 16" of precipitation each year (with most falling as snow) didn't qualify her as a desert gardener.
With humidity often below 20% on summer afternoons and daily temperature swings of 25 to 40, Kathie's part of the world sure isn't PNW maritime!
The "garage tent" suggests a fairly large frame for a greenhouse. You've got lots of air volume in there. I bet you could do like Eliot Coleman does in his
Maine greenhouse and set up low plastic tunnels inside to protect plants from the cold. Two layers of glazing for the sunlight to pass thru but a fair amount of overnight, cold weather protection!
SHF's 6x8x7 is about the size of my 1st backyard greenhouse. Like Marshall, I'd worked for awhile in a large commercial greenhouse. My utility porch windows were darn near perfect for starting plants but just not big enuf! So, I built a lean-to greenhouse against the southwall of my garage. What a wonderful start to the growing season

!!
I had a wide bench the length of the greenhouse and used it for flats of plants. I'm still using that approach in one a little larger. Kathie might be better off getting things right down on the floor (or ground), I'm not sure.
Summer ventilation is tricky, SHF. It is good that yours has 2 windows because on real warm spring days, you will probably want it nearly completely opened up. I had 2 windows and a door at 1 end and that really provided enuf for the sun-heated air to escape, even against a south garage wall. The wall really helped to hold heat but I found that I couldn't get much use out of the thing during winter months without pumping an awfully lot of $heat$$ into it

! Since there was a small room - complete with a chimney for a stove - on the other side of the wall. I'd thought about cutting a window in the garage and . . . . but, that would still take a lot of heat and I never quite got a round tuit . . . Then . . . I moved - pulled down the greenhouse & took it with me!
I think that besides having MORE - what a greenhouse enabled me to grow better was peppers! Shoot! I remember setting out pepper plants that were only about 4" tall back, pre-greenhouse. And eggplant!?? Forget-about-it! Of course, these are heat loving plants and I had to provide supplemental heat - or, carry them back into the house every night. Yeah, I did a lot of that -- carrying things out of the greenhouse at sundown and carrying them back in after sunrise. And, sunrise doesn't mean - instant warmth! The things are just able to hold
SO much heat with all that open-to-the-elements that is necessary for gathering light during the day.
Coleman does this - cool-season greens in the ground and covered with a plastic tunnel, inside the greenhouse. He called his book Four Season Harvest, NOT Four Season
Growing. The winter crop was essentially grown during the warmer weeks of the fall and harvested during the cold weeks. Just an idea . . . and you've got JackB here to tell you about season-extension of warm-season, indoor crops like tomatoes and peppers

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Kathie . . . this rain . . . it's turning to snow . . .
Steve