I mean technically they are optional but I think you are somewhat underselling how shit it all really is. For one you can fuck up and skip whole quest stages that tie back to other unrelated quests by simply going to the Black Keep too early. You know the one big black building on the horizon that your camera ever so slightly defaults to when you respawn. In my case this fucked up the dragon form quest because I could not get an item from a different quest that was literary skipped by going too close to the keep.
Further more the quests that this did not screw I almost ended up skipping because From still requires reloads or boss fights(I am entirely unsure which it could be easily both) to unlock the next quest stage. So I would have a chat with one of the NPCs, they would start repeating lines and I knowing that I had already skipped about 1/3 of their quest content by going to the keep too early left them. I assumed since the skip broke the dragon quest that it also pre-emptively ended the other quests as well. But later when I was fucking around it turned out that in fact new dialog was unlocked and the quests or at least some of them were finishable.
Problem is that was nothing but dumb luck on my part and had I not gone back to that area to test some weapons I would have wholly skipped those quests and most likely made the second to last boss battle even more of headache than it was. I can easily imagine most players completely skipping these quests by accident without even knowing what they did wrong.