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One of those, with a shorter seat binder bolt hanger than the one you had just now?
https://www.rivbike.com/products/nitto-brake-hangers-key-fobIt won't look good or work very well but this is the bodge thread.
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Garmin 1000 with openfietsmap.nl maps on which have cycle paths marked, OsmAnd on the phone for when I want a bigger overview (get it off fdroid for the full version without in-app purchases, download all the relevant maps before you go, https://f-droid.org/en/packages/net.osmand.plus/ ), plus a vague idea of the direction I'm going in so I can use the local street signs. French cycle route signs are green and white like this: https://en.francevelotourisme.com/tips-and-advices/signposting-and-marking-of-cycle-routes-in-france
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What brake pads should I use? I have these and they're fine in the dry but in the wet they're shite, they make a noise but don't slow me down.
https://www.swissstop.com/brakepads/discbrakes/disc15/disce/
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You're more likely to take far too much than too little. Make sure you've got puncture repair stuff and at least some tools. Cable ties. Big strip of gaffer tape, you can wrap it round an old credit card to save space. Some M5 screws so you can fix your rack.
You're not going into space so anything that you do forget and really need you can still get.