In case it helps anyone else I think I worked out why my phone stopped booting after updating. Originally it was running the /community/fredldotme/devel channel. I think I must have accidentally switched it to /arm64/hydris/devel in the phone settings playing around one day. In the phone they both show up as Development but in the installer you see the full path and can tell them apart.
Not sure if this was the problem, but I assume swapping between devel channels is not a good idea!

Thanks for the awesome work @fredldotme