Re 4-bay NAS and filling 4th bay, which setup

Thank you so much for these excellent videos! I bought a 923+ recently and have 3x 4TB SSDs in it now. I bought a 4th to either expand capacity or switch to SHR-2 to have double redundancy just due to paranoia, but really I don’t need more than 4TB of critical data to store. I thought I heard you say don’t fill up all the bays if we don’t need to; instead, leave one empty. Why is that?
Also, better to keep SHR? Move to SHR-2? Or just create a separate volume?

You sum up pretty much all the technical possibilities of your hardware at hand. The only question that is relevant is ‘What do you need?’.
Your needs dictate the best solution, and you do not need more than 4TB to store.

Of course, you do have your 3-2-1 backup plan in place, do you? That would be my primary concern, not SHR versus SHR2.