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I used something similar for a few rides this year (an old classic light which I owned for years but never got round to using as I hate stuff on my back) but it still feels like a backpack. Tried my girlfriends Camelbak Chase Vest. The vest style packs feel way better so got one of those in the end. Ended up getting the womens version as a place was selling samples on ebay cheap.
https://www.camelbak.co.uk/packs-c36/bike-c50/womens-chase-bike-vest-4l-with-1-5l-reservoir-p15
I've had a rec from someone else for this one:
https://www.camelbak.co.uk/packs-c36/bike-c50/hydrobak-light-hydration-pack-2-5l-with-1-5l-reservoir-p35
someone else uses it on road and says it was well ventilated and he preferred not having anything on his waist.