DD1 was in Grobags from about 10 weeks and loved them. We were heading into summer then (in WA) so I bought a couple of 1.0 togs and a 0.5 tog in the 0-6mth size and that was fine. In the 6-18 mths, I bought two of each (.5, 1.0 and 2.5 togs) and found that perfect too. We then bought the same in the 18-36 mths.

DD2 wasn't a fan of Grobags early on so we used the Love To Dream swaddle bags until 4-5 mths. She loved them so much that it was really hard to wean her off them! I ended up buying Sleepy Wings to use with our Grobags which worked quite well. She finally weaned from her 'wings' around 7 months, I think?!

Anyway, the point of that little story is that not all babies like Grobags, lol! DD2 hardly wore our 0-6 mths bags. So I agree with OP, I'd buy the next size up and get 2 each of the 1.0 and 2.5 togs. Depending on whether you use an aircon in summer or not will determine if you need a 0.5 tog or not I think.

Just to throw a spanner in the works, look at different brands too. Dream Bags are much cheaper and equally as good and beautiful. Living Textiles are often cheap through their website. And for an absolutely gorgeous, snuggly winter bag, try Ergo. OMG, they are divine! I bought one 3.0 tog for this winter and it's just beautiful. DD2 also had an Ergo Cocoon in the 0-12mths size which was great for transitioning as it has press studs on the arm holes.

I'm a bit of a sleeping bag addict