No direct experience with the REI bivy, though I have a general impression that their house brand gear is fairly high quality and function for the price.
Been a bivy fan for a while. I started with a GI goretex bivy decades ago. Bombproof & 2#. I have use SOL (good in a pinch) OR and a few others. Bought a bora bivy last year and haven’t used any of the others since. Great product. Great service. I would definitely check them out. EmIl them your specs and usage and they will recommend something.
If you go the REI route or any bivy in general, watch your weight. With a bivy, it is easy to add a ground cloth, overhead cover and bug net and come up with a total weight that is actually higher than a small tent, especially a really light weight one. Dyneema and modern construction methods have made solid, weatherproof 2# tents a reality. For my preference, I actually prefer a smell mesh walled tent in hot, buggy weather. I also like a tent in deep cold or when I expect significant rain. I will take a bivy for anything in between.