Big Sur 11.3/4/5 not installing on MacPro 6,1

I've tried waiting out for a fix from Apple for this but 11.5 just dropped and still cannot complete install. Running Big Sur 11.2 just fine. Cannot update past that.

I've downloaded the updates (via App Store as well as System Preferences) 11.3, .4 and now .5 and they appear to complete. I am then the dialog box gives me the option to install and then I am trapped in a loop where I click Accept. It blinks and gives me the option to click on Accept. It blinks and gives me the option to click on Accept. It blinks and gives me the option to click Accept. . . . etc.

I've tried downloading and installing in regular and safe mode but there is no difference in outcomes.

I have plenty of space to install the update (160GB free). I have internet and no other weirdness.

Any ideas good people?

MacPro 6,1 - 3.5ghz 6-Core Intel E5

SSD: XPG SX8200 Pro 1TB Internal PCIe

RAM: 48gb 1066 DDR3

GPU: AMD D300