Sleeping systems - bags, pads, matts, liners

I looked into the fabrics a year or so ago, which is a big concern due to breathability constraints. EVent seemed to have a reputation of being actually kind of breathable (more or less everything else is not), likely because it doesn't just pass vapour, but also air. Con is that eVent seems not to be very durable. You might find some old Rab bivies with that fabric. However, those models switched to pertex something, which is also what OR helium uses, and people review them as being ok wrt breathability.

Breathability is a weird buzzword and I wouldn't trust it too much. My OR Helium gets very muggy, but as does everything else including my half-mesh bivy. I live in Scotland and don't bother much with site selection, tho. I chose the OR helium because of the kinda breathable reputation, the hoop, and the mesh. The use case for these things IMO is to mostly use them open, and close down only when necessary. On first glance, it's not the lightest option, but my relatively minimal flat tarp + mesh bivy setups tend to be a few hundred grams heavier due to poles, stakes, groundsheet, etc. I'm not concerned with weight here tho, but utility - a bivy is easy to pack and setup in a crap campsite. I also like feeling abit more open vs. closed down in a tent. Ideally I'd cowboy camp most of the time, but that's a non-starter in Scotland.

I don't like Alpkit's elan thing, last time I checked it seemed heavy, contrived, and I expect, not very breathable.