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I'd like to see OS maps onscreen ideally and be able to overlay gpx on these. Strava compatibility nice to have but not essential.
Usage in the short term: coast to coast (road bikes) and lord of the loops (mountain bikes), so battery life of a full day is necessary plus good water resistance as I only seem to adventure in the rain
Don't really want to spend more than £100 second hand as that's the budget I told the wife
Is it possible, hive mind?
After extensive experience I've got rid of all mine.
It's more fun getting lost.
I'd argue that you don't want actual OS maps. The level of detail is a hindrance on the move. Both Garmin and Wahoo now have very good high contrast maps which are really easy to follow. You can use a phone or actual map for backup, but I find it rare that it's needed.
Battery life on the current models is 15-20+ hours and you can charge on the go with a power bank. If you're buying secondhand I think there used to be issues charging older Garmin while in use? Some of them had poor battery life too, so look into whatever model you're thinking of buying. Any Wahoo should be fine.
All of them should be completely weatherproof, that's what they're built for.