Copy an external Time Machine HD to an external SSD.

Thank you for the explanation. I think you have overthought this issue. When you get the new Mac, I would use Migration Assistant to transfer the data I wanted transferred to the new computer. It's easy to do. Carefully read and follow the instructions. Then you can set use Time Machine to do its backups to the new disk. M.A. will not delete any of the data from the old machine. you must do that yourself. I hope this is helpful.

I had trouble with my TM drive a while back. Since I had no files on it that weren't on my boot drive I went ahead and erased and started over from scratch. No problems since then.

Not the same issue as you're having. So keeping the rotational TM drive and starting afresh on the SSD would be my plan of attack.

See Kappy's post in this topic: Moving Time Machine backup to new drive

Please explain why you feel this a good idea to do. It will take up a lot of space on your hard drive and would serve no purpose that I can see.