One advantage to keeping with the tarp/bivy combo is that if you need to do any maintenance on your bike while it's raining, you can use the tarp. And the bike can be all or part of your pole system.

Also, you can cook under a tarp more easily than a tent, in the rain. Of course, be extremely careful. Some materials do just about disappear when touched by a flame.

A light, water-repellent bivy will pack pretty small and light, but as stated it is not easy to change inside of one in case you need privacy. And you'll hear the bugs right next to your face, so if that's a concern...

If claustrophobia is a problem, you might check out a 'bothy', which is basically a two-person bivy/wind shelter for mountaineering. Exped makes one that packs down really small and can be staked out like a tiny tent, but you would need to either go with a head-net or build a door with a piece of netting to keep the bugs at bay.