If OTC continues to fail, there is an unofficial solution called the Semi-Offline Update Project.
https://forum.xda-developers.com/windows-10-mobile/guide-win10-mobile-offline-update-t3527340
Carefully follow the steps and it should allow your Windows 8 phone to update to Windows 10 without erasing your apps and data.
This is a third-party method that's unsupported by Microsoft so obviously use at your own risk.
My Lumia 640 with Windows 8.1 was unable to upgrade with OTC but successfully upgraded to Windows 10 with this tool. After that, I was able to receive normal Windows 10 upgrades up to version 1703, the final update for my phone.