Rain won’t fk any half decent sleeping bag. Like ma said, even wet it will still perform to some extent. Down clumps together, is a bugger to re-loft. Hollow fibre stays in siitu. I send my down bags off for washing. I liked your idea of a tarp or bIvi for protecting any sleeping bag from the rain. I would always say buy the best you can afford that is a) big enough for you b) a bargain you can’t turn down. C) look after it.
Theft is a big concern, your life is in your one sack. A rain cover can hide zipped pockets, pacsafe https://www.amazon.co.uk/dp/B0…hvtargid=pla-336986965606 can let you relax when your pack is stowed in bus holds, on trains, in hostels, or when rough sleeping. I have a few and they can be found much cheaper. Don’t trust folk with your kit. Turn your back on it and it’s gone. Especially new gear. It’s a target.
Sleeping bag Zips can fail. Draw cords can make it difficult to escape if attacked, robbed, emergency.
Having a full length zip sleeping bag inside a bIvi is pointless.
Years ago explorers went with canvas over wood or metal framed rucksacks and wool sleeping bags. Technology and design has ironed out many problems and yet its fashionable to strive for the latest development to gain an edge.
Most of the gear out there, including Tesco’s own range is a far improvement on what was around and affordable 40 odd years ago. Learning what’s essential and what’s desirable is a personal journey. You have bought some good kit Butch, kit that will work for you long after your trip.