Second Charmera arrived, and I got the other naturally coloured one, so that's a win!
I got the second one so that I don't feel like I need to be careful with the first one, so it's the sort of redundancy that will let me push for getting shots even if it's raining or I'm in a crowd or whatever.
I found a second microSD card for it, but it had issues, and it turns out that if you format the card in the camera it stops being functional - either for the camera or for reading it on a computer!
This ended up being a win though, because I discovered that if I format the SD card to the oldest format FAT32 IIRC then it works better than the previous format (ExFAT) which had random stutters in the footage. So after some confusion and a bit of trial and error that's a big win! It still duplicates every third frame though, so it didn't change that behaviour.