Your expectations around the iPad are a bit much. Yes, SnapBridge and the like are painful, I would recommend grabbing a card reader and directly working against the storage device.
iPad's are great for a lot of things, model contracts, shoot notes, portfolio display, etc. Wireless tethering is slick, especially the latest models that are powerful enough to do it - remember that the D850 is from 2017. The latest mirrorless bodies do a much better job.