Cheers!
I had tried replacing the -ct version with the other versions, and it hadn't worked, but I tried again thanks to your advice and that fixed it. I guess I must have forgot to reboot, rebooted the wrong router, or done something else equally stupid.
Mesh starts up ok and is working using ath10k-firmware-qca988x, which installs /lib/firmware/ath10k/QCA988X/hw2.0/firmware-5.bin, in place of firmware-2.