I am looking for the lightest setup possible for mostly dry summer time weather conditions.

So Ideas - Assuming air pad (8oz) & quilt (22oz).

1) Tarp/Fly with ground sheet.

2) Waterproof/weatherproof bivy only.

3) Tent only.

Considerations:
Tent and tarp setups provide (some) weather proofness but may be heavier that a bivy only.
Tents / tarps need tie downs and guy lines.
Bivy may be lighter but is open to the weather.
Bivy doesn't need any tie downs or guy lines.

Have you had experience using these types of setups with the most ultralight options?
Experiences?
Recommendations?

It seems that there is no perfect setup - they all have consequences and drawbacks.

At the moment I am inclined to just go full cowboy with a bivy only, but I don't want to get stuck with that if it's a bad idea.

Thoughts?
Thanks!

The original bivy sacks were intended for exactly what their name implies, a bivouac. That meant providing the barest means for surviving whatever conditions forced you to stop and seek shelter. It was gear designed for infantry on the march during war or mountain climbers waiting out the hours of darkness on some cold, miserable ledge, not gear designed for comfort or convenience.

As soon as you add more conveniences, like a tarp or some pole structure to the bivy, the weight savings compared to a UL solo tent start to vanish. The one thing a bivy sack is truly superior at is adapting to the needs of a bare bones bivouac, wherever you find yourself needing shelter, including places a solo tent couldn't even be pitched.

If you are inclined to cowboy camp most of the time with just a groundsheet under you and the stars above, and only need the bivy occasionally and are willing to put up with all the discomfort of a genuine bivouac, sleeping unsheltered except for crawling into a waterproof sack, then that's the lightest setup you're ever going to find. You can try it out and see if that works for you and the style of hiking you want to pursue. Most people end up gravitating to an UL tent.

P.S. Claims about a bivy's breathability should be heavily discounted. Expect condensation.

You hit the nail on the head.

If you have a 10-day forcast with no precipitation predicted and you are only going out for a couple nights. The Bivy seems like the right solution. Its only their to get you home if everything turns upside down.

If you are going out for an extended time or there is a chance of precipitation predicted, I would lean more towards something with a bit of comfort. Tarps are light with lots of options, but if you add in an inner netting (for bugs) and ground cloth (for a clean place to sit) you've basically made a tent and given up all of your weight savings.

If you are talking about buying equipment and want to only purchase one of the options, it becomes impossible to predict what the weather is going to be like on your future hike. If you are flexible and have an open calendar you can go with the bivy and just cancel if the weather doesn't line up. If you tend to have a fixed schedule I would recommend something like a tarptent.

One thing to remember about a waterproof bivy is; if it rains or snows, the cold water is right against you, whereas a tarp or tent gives you a buffer.

I tried to figure out a super light setup several years ago, but since the wind here tends to swirl and change direction a lot, I ended up with a 'mid. Sometimes I put a splash bivy under it, sometimes a net tent to give me a mosquito-free space.

I have had some experience with all types of setups. It always comes down to expected conditions. A tent or bug tent is necessary if you expect swarms of mosquitoes. Otherwise I much prefer a tarp over a tent for freedom of movement, storage space, and ventilation. If you take only a bivvy and it rains you definitely need a tarp because you would otherwise be trapped in the bivy bag and can't cook, sort gear, change clothing, pee, read or do anything without getting wet. So if there is [/b] any chance of rain[b] take a tarp with the bivvy. I used a Mylar"space" blanket for a ground cloth last time I was out. It got a few small holes in it after 2 nights, but worked... sort of. A lightweight plastic painter's tarp is more durable.
If you are inexperienced it would pay to take a little extra gear but see what you can get by with on a couple of weekend trips.

As others mentioned, I think you should consider bugs and mosquitos, in such cases a tent is very useful, it also keeps you dry on rainy days. There are some ultra light tents you can use. Also the tent protects you from the sunlight on summer days.