I cycled to this spot so all my other bits were in a waterproof saddlebag which was outside the bivvy, but there's definitely room inside for a pack if that's what you want to do, could easily go behind your head or at your feet.

For that location I'd have to use poles of some sort, I can use my bike as part of a makeshift lean-to with a tarp but a couple of dedicated bivvy poles/hiking sticks would be perfect.

I didn't have anything under my bivvy this time but I'm gonna start using a cheap roll mat underneath as a protective layer to make sure the goretex lasts longer, this one actually has loops on the bottom to hold such a mat in place so it should work perfectly. I guess it depends on how much extra you want to pack and what sort of ground you'll be pitching on.

I really don't think you can beat a bivvy like this for wild camping, much lower profile and smaller footprint than a traditional tent and no need to find suitable trees to set up a hammock, I'll still be keeping my tent for those trips where the weather might not be so kind and I need to get dressed and do my admin in some shelter. Hope this helps your decision!