As far as I can tell they've removed the ability to turn off real-time scanning with group policy, so you have to disable the entire thing and not get on-demand or scheduled or download scans.
Defender will disable itself if it detects another AV product is installed...maybe someone should make one that acts as a no-op AV scanner.
Seems to be working for me still, though I set the GPO back on Win10 and it carried over to Win11 through an upgrade. I see some reports of needing to disable tamper protection first but should still work.
Ah yes, you need to disable tamper protection as well. (Which is kind of strange... if a virus can disable the first one can't it also disable the second one??)