Can't progress Monterey install... or turn off FileVault

I have a MacBook Pro M1 running the latest version of BigSur and am having trouble updating to Monterey.

When I run the Monterey installer, it shows the hard drive it's being installed on and I click continue. I enter the computer password in the box that appears but when I submit that, it just stays on the same screen and doesn't progress the install. If I hit continue again, the password box re-appears and I'm stuck in an endless loop.

Some things to know:

• I've downloaded Monterey 3 times but it's the same every time
• I'm attempting this update from an Administrator account
• I read that turning off the Firewall may help but it made no difference
• I read that turning off FileVault may help but it doesn't turn off. After I unlock the padlock on that screen click the off button, the button itself doesn't toggle off
• I tried creating a second administrator account and redownloading Monterey but the problem remains

I've updated OS's on dozens of Macs over the last few decades and have never encountered anything like this so hoping the community can help me out on this one.

Thanks.

Herc.