I believe this is the solution if anyone comes across this in the future! Haven't tried it yet will update it:
"Your OS partition is MBR, which requires the use of CSM (compatibility support module) to boot from it, rather than the modern GPT layout. When you enable resize bar, it disables CSM, since they're incompatible, causing it to boot back to the bios on reboot.
Go to the boot tab and enable CSM and you will be able to boot normally again but no resize bar yet. You'll have to convert your OS partition to GPT and then disable CSM again for it to be able to function with the resize BAR ability. Convert MBR to GPT. Warning there is a risk of data loss, so back up anything important first. It's rare, but can happen.
Note: if you've only ever signed into Windows with a MS login, and don't have a local account on the PC, you may need to go to settings>account and click on the "sign in with a local account instead" option to convert your account to a local account, or you may be unable to access the command prompt while trying to reboot or be unable to run the mbr2gpt tool due to insufficient privileges. You can convert back to a MS account once completed.
I'm posting this here as a PSA so I don't have to keep responding to the influx of tech support posts"