I just got done doing something very similar. Here’s what I did:
I created a single IK Rig for my main skeleton. I then opened up this skeleton and went to Window->Asset Details and added the other skeleton as a ‘Compatible Skeleton’. If you do this for all the skeletons you want to retarget to/from, you only have to create the IK Rig once and you can copy it for any other skeleton (that is compatible) - just switch the preview mesh for each copy.
Also, I’m no expert, but I’ve had great results with just creating the most obvious chains and leaving the rest alone. Arms, legs, spine, etc. you need, but sometimes twist bones, IK Goals, etc. are overkill.
Hope this helps.