I'm also in zone 8 and have been redoing a hoop house style greenhouse that was here on the property when we bought it. When we bought the property it had no ventilation, and contained just three raised beds. I removed them, adding a couple of automatic vents, added some thermal mass. I'm growing in large containers at this point. We have very bad ground squirrel problems here, so I lined the floor with hardware cloth so as to keep them out. The ventilation was still not enough for our hot summers so I added secondary doors covered in hardware cloth. Even still, the only things that I can grow in the greenhouse during the summer is sweet potatoes and okra.
The summers here continue into the fall, so the greenhouse doesn't really help much in the fall. I do grow lettuce all winter long, fennel and beets do well also. And it's great for a spring crop. I can plant seedling in March in the greenhouse for early crops of zucchini, peas, cucumbers and probably a few other things that I'm forgetting.
I live off-grid, so the greenhouse has no electricity. I don't actively heat or cool it. And I have no intention to do so. I really find it useful in this climate.