I’m not sure about free options, but if you are using Mixamo an extremely popular option is Mixamo Animation Retargeting 2. You will still need to make minor adjustments to the bones to make them perfect, but it does most of the heavy lifting.
I’m not sure about magic tools other than unreal. If you prefer not to use an external program, you may want to check all the settings you can adjust in your IK retargeter. I assume you are using one without getting the exact results you’re after.
Magic, no.
I show the process of going from one pose to another in blender withn this video:
Probably painful to watch if the pose trick is all you want out of it, but still… its a practical application.
As far as magic tools…
The Rokoko live plugin for blender offers a retargeting system.
Perhaps you could momentarily levarage that to transfer the pose from whatver into the character, so that you can perfom the trick explained in the video to set the origin pose.
Mixamo animations are small part of my animation assets. I’m not sure the plugin would fit the other animations I got from other places like UE market.
You know, you may not need to.
Have the engine retarget them, and then export them again.
The end result should be a set of animations that does not require the tpose to start.
Then in the clean project you can simply import the last exported animation, and it should just work without causing a mess…