Shift all content on every page down 5mm OR adjust bleed settings?

I am working on a 5" x 8" book that has a 5.125" x 8.25" trim size with the printer.

For bleed settings, then, I have 0.125" (3.175mm) on the top, bottom, and outside, and 0.0" bleed on the inside.

My book has some special circumstances (replicating something that needs to remain a certain size because it is "real" size), so I don't have flexibility really in terms of the total size my content takes up. Including my page numbers, I currently have about 5mm of space at the top above the page numbers and 13mm of space at the bottom below the text. However, after a couple draft prints, I realized that there is some fluctuation in the cut so that in some prints I get 1mm of space at the top and then about 18mm of space at the bottom.

I am trying to figure out what the easiest way would be to basically move all the content of every single page (images/text/etc.) down about 3-4mm so then I can have around 8-9mm at the top and then 10mm at the bottom.

I thought maybe I should just set the bleed at the bottom to 0mm and set the bleed at the top to 6.35mm, but that seems like a not ideal way to do it.

So my question is: Is there an easy wa