How to work with heavy character rigs?

My problem is i stumble upon some of 3D characters which are intended to be used in renders, models themsef are simple, but because of their custom rigs with lots of shapekeys and different settings, i can barely move them.
I just don’t know what to do, should i spend time and recreate those characters with simple shapes, then find their rig (one or two of many), place it in separate file, animate it and then copy anims to original character? Or maybe make duplicate of file with character and then somehow simplify character, which will 100% break it and it is not clear will it move properly after our not.
Both variants are very time consuming and exhausting.

Any thoughts?