Get the best you can stretch to and then some. Weight and packed size are paramount for your biking bivvy trips, you have to be able to get everything you need in your ruckie. Small, light and warm = expensive! Mountain equipment, rab, etc but your talking £150ish, dont scrimp though you’ll regret it. Alpkit pipedream might be worth checking, got one of their skyhigh bags for winter which is very good, well made just too bulky for the bike. Forget the idea of getting a bag that will be suitable for both your needs because you’ll end up compromising both.