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That is in line with what I thought, and my consideration of the carradice zipped roll (which I think hangs lower, but I have stuggles to find pictures on a drop handlebar bike) I may also look (for the price) at
https://www.decathlon.co.uk/100-bike-handlebar-bag-25l-id_8383692.html
or even
https://www.decathlon.co.uk/junior-front-bag-pink-id_8380516.html
Both of which look as if they could give a couple of extra litres space.I take it your suggestion also just hangs below the bars? Do you have any issue using the tops?
Now new Tiagra is internally routed there is also the option of new shifters. First step probably to see how much I can get in the SWR tout.
You'll probably struggle with a Klickfix mounted one - the few I've used all project far enough forwards that they'd be in the way of the cable coming out the shifters.
Just got one of these though (I love it!) and pretty sure you'd be ok with it: https://www.ortlieb.com/en/Accessory-Pack/