About Miraak
I don't have DB. I only have UESP.
"He cannot be ragdolled under any circumstances, but can be staggered." - plenty of possibilities. So I'm only thinking Force Without Effort, Impact, Power Bash, Power Shot, Frost Atronach, Unrelenting Force. Plus anything that DLCs have in addition to these.
Personally I didn't find the fights challenging at all. And yes, my potions would run out faster, but I rarely use healing in a fight, instead I try to avoid taking damage in the first place (eg block or dodge attacks). 'Legendary' doesn't make the game harder for me (I had always played on master, from level 1 onward), it just makes the combat longer. Some fights taking longer I can understand, but several strikes just to kill a wolf or a skeever is a bit silly to me.
I too go for quality over quantity, but Legendary difficulty isn't 'quality', imo - it was a lazy way for Bethesda to have a new difficulty. Now what I would've like to have seen were more opponents added with difficulty, not some artificial damage adjustment.