I have a Palram Snap n Grow, it was $600 at Menards 10 yrs ago, still holding up very well, great starter 6 x 8, has clear plastic it's just dew on windows in photo
When you buy a steel tubes strong carport to convert it into a greenhouse it only cost $100 and $120 for the UV stable greenhouse plastic. Labor not included. And extra woods 2x4s to make it wind proof not included.
10x20’ is still too small: but putting 3 next to each other is possible 20x30’
I’ve seen a couple of Harbor Freight recommendations. My neighbor just bought one from Harbor freight and is not at all happy with it. He is having to do many modifications to make it work. He highly advised against the Harbor Freight GH.