I don't get it.
Nightmare. If you change your tactics then fights aren't long and tedious. Its totally right to say you don't have to change tactics, but doing so does make encounters on nightmare trivial, so...change your tactics for nightmare.
I'll list some examples even.
liberal use of potions, like mighty potion.
Stronger synergy with companions, like increasing everyone's crit chance while using varric.
Making better use of special skills, like using dispel on barriers rather then trying to bull through them.
Making better use of combos to eat through enemy health faster.
Making better use of the crafting system to make better gear.
Using fade-touched gear with skills that proc multiple hits.
Skipping most the fights early in the hinterlands to quickly grab the forces agents necessary to get advanced schematics.
There are many many things in this game that just chew through enemies so fast that unse you know of them nightmare becomes a requirement just to have a meaningful game, and even with it some things must be turned down or ignored, or fights end before they begin.
@op
Its impossible to give any meaningful answer. The difficulty your comfortable with depends on how well you know the game. Do you know where to find good schematics, where to find good materials, do you know what skills work well, do you know what to expect during encounters. Do you know where to find the good potions, where to spend your inquisition perks, what team compositions work well together, where to quickly grab agents, and so on.
Very little of the difficulty is dependent on how you play, its much more to do with how you prepared before the fight.
Finally, as a completionist, your not going to enjoy this game. There is a ton of fluff in this game, and enemies only partially scale. Most the quests suck, and bring the game down. You'll be over-leveled, and never leave the hinterlands, and you'll be complaining about how terrible all the quests are. Most the loot is crap. Its so much more enjoyable to play through the content that the staff actually put effort towards rather then playing through every crappy fetch quest you run across. I ca't stop you from playing how you want, but I don't recommend it.