See if things work better if thr Mac can be booted in Safe Mode. If there are kernel add-ons around, those won’t be loaded. That’ll help determine if this is a more general problem, or maybe a problem with add-on software,of some sort...
Use safe mode to isolate issues with your Mac - Apple Support
If you’re able to start the upgrade again and have previously installed any add-on security tools, anti-malware tools, any performance-optimizers or cleaners or other kernel-based extensions, try removing all of them per vendor instructions, and try the Mojave upgrade again. That’s if you have a way to start the whole process over again, from whatever macOS version was in use prior to starting the Mojave upgrade.
Failing that...
Create backups all user partitions present on your boot device, create a bootable Mojave installer on a scratch USB device, boot that, wipe the disk, install Mojave, and migrate in your data and apps. Basically, start over again, with a clean install of macOS Mojave, and let the installation or the Migration Assistant tool transfer over your data.
How to create a bootable installer for macOS - Apple Support
Or yes, check with the folks at the Genius Bar. (But you’ll still want to have backups, before starting down that path...)
