Surprisingly, quite the opposite happened. As other users pointed out, Windows 10 version 2004 onwards seem to have Window Manager\Window Manager Group already added. In an attempt to try to re-add, I removed it. Big mistake. I was unable to add it back.
Searching online I came across a way to add it back by exporting the security settings as an INF file, appending "*S-1-5-90-0" wherever necessary, and importing back into gpedit. It unfortunately worked for all other entries, but not for "Increase Scheduling Priority".
Instead, this seems to have the opposite effect. I no longer face the screen flicker issue while using Windows INK Workspaces such as OneNote, Snip & Sketch, and Whiteboard.
This does work and persists after multiple reboots. I wouldn't necessarily call this a "Solution" since I have no idea how or why the opposite to the advice provided worked, but it did for me.