This is how I fixed all my issues - freezing, crashing, black screen, unable to be heard on calls etc
- Tried to downgrade to 10.3.3, this kind of worked but all my downloaded apps had a missing-icon icon and didn't open. I tried to fix this and rebooted 2x. On second boot the phone wouldn't turn on - stuck on apple logo.
- Went into recovery mode with USB plugged in. iTunes informed me I needed to "update to 10.2.x". Tried updating, said something like "declined to update this image on this device"
- Updated iTunes from 12.5 to 12.7 and made 1 last attempt to recover phone. This time it prompted to update to 11.1. It worked! Booted up but I still had to manually delete all the missing apps and re-install from Store.
Since doing that I haven't had any issues (so far anyway). Everything is running smoothly now without the massive mem leaks 👌
tl;dr if Factory Reset does the same thing then try that. Otherwise go into DFU mode and do it through iTunes. Another user said he tried this but it started to go wrong again, so only more time will tell but looks good so far.