How to disable "Eject device before disconnecting or turning it off." finder notification
I don't think you can eliminate this warning.
Normally you shouldn't even think of eliminating it, but I can see your use-case for not needing it for this specific device.
I would be more inclined to prevent the volume from mounting in the first place, then it will never 'not be properly ejected'.
You can prevent it from mounting using fstab.
I wrote an earlier answer on using this, so I'll link to that rather than covering the same ground again. The question itself is entirely unrelated, but the answer contains step by step for fstab
I do not know, however, if this still works as advertised in Big Sur, with the protected OS partition. Someone else would have to help you through that, if it's not mutable any more.
Time Machine restore to new partition, different computer