You can skip posing the character, but you have to pose the metarig as it serves a dual purpose of posing and modifying the metarig.
As it stands right now you can not safely use your original metarig to rig your character. You have to create a new one. If it worked for you in the past consider it a fluke because it was not very different from the original and everything the script expected to find was at it’s default place and setting.
Most of the characters I use are auto-generated without any bones or are from marketplaces with just skeletons not rigs. So rigify pre-built characters are not a priority for me.
Export your rigify rigged character to FBX with only deform bones and re-import it back to Blender. This will get rid of the rigify control bones and you end up with just another skeleton that already has weight painting. After that re-name and re-rolls the bones. With the new tools in the latest version this is easy. And continue with the tutorial to finish the process.
In this case you do not need the Unreal Mannequin. You can SKIP posing the character. You will end up retaining your original weight painting and your original pose. There might be some adjustment needed for the twist bone weight paint as Rigify and Epic Skeleton create them at different locations.
There is no need to involve Mixamo in this.
Perhaps I’ll do a video on this process too. rather occupied at the moment but hopefully will get back to making videos.