Some of the things in this thread are very confusing to me. In post #1 you said "everything is hard-wired." That phrasing generally means that the backup drives are directly attached to the Mac by USB cables. But the MBP accesses the TM drives over your local network, which uses SMB protocols and is NOT hard-wired. And from your description, when the MBP sleeps, that connection is lost.
I had thought about this solution too. And I am/was not sure it would work on my external SSD. Remember the SSD is connected to a Mac Mini in the network. The Mac Mini never sleeps. And what is more, I use 2 SSDs to make TM backups for my M1 MBP. Let’s call them disk 1 and disk 2. Disk 1 never causes this problem; it is always disk 2 that gets the error and that without exception. That is why I did not implement the solution I thought of, and you suggest.
That paragraph is very confusing to me. You seem to have three drives involved. One is an SSD attached to the Mini but it is NOT clear if it's really attached or "in the network," whatever that means. Then you have two more SSDs for backups of the MBP. How are they connected? From Post #15, it seems to be that they are somehow in the network and are attached as network drives, NOT directly attached. And no clarity on what disk 1 or disk 2 might be, or where they are attached. I'm totally lost. You had said that the drives were attached by USB-C to the Mini, but then said they had IP numbers, which implies network attached.
So, can you start from the beginning, please, and describe exactly what you are using for the TM backups? What drives, what are they attached to, how are they attached, how are they connected (what protocols), how is TM set up, etc? Remember, we cannot see the setup, so you have to describe it as if we are blind.