Tarp, DD 3x3m is a good option. They’re pretty tough and offer reliable shelter. They do an silnylon ultralight which is more expensive, but in my opinion, perfect for your needs due to the weight.
An essential item you’ll need in any situation is a sleeping mat. I’ve had a few self inflators lose air overnight due to small punctures, but Multimat use a decent foam in theirs that’ll give you some cushioning and insulation in this disastrous situation, and aren’t too expensive. I carried mine under the lid of my rucksack with the end sticking out and must have caught a bit of bramble or thorn. Lesson was learned, pack it in the middle of your bag surrounded by other stuff. Blimey, stick your saucepan on one end like a hard hat
Of all the things that can go wrong, it’s a bad one. An Alpkit also deflated overnight and was about as much use as a couple of carrier bags under me. I think they’ve changed the foam density now, they’re a very responsive company.
Other option is the closed cell foam roll-mats that are light and cheap, but bulky. No puncture worries though.
The DD 3x3 tarp offers some good options for set up.
I’m not into bivi bags too much, but have had an Alpkit Hunka XL and a Brit Army goretex. I sold the Hunka, kept the army. It’s wider at the head end and easier to get into, it’s a good windproof addition for hammocking if the night looks to be cold or the breeze is getting up, but I’m not a ground dweller unless I’m tenting it. I just don’t like slugs and beetles in my sleeping bag or on my face.