No this is Patrick

660s are less like the 600 than the 6xx/650 but if you dislike into intimate presentation and lack of sub bass I would recommend a different brand. Sounds like you'd enjoy a hifiman, start with the 400se and see how you go

Your assumption is correct, at least according to my audiologist. She says it doesn't matter as long as the volume isn't too high, of course iems can also cause infections if not properly cleaned etc.

Awesome, glad that's helped!

Closed backs are not necessarily more fatiguing than opens, it's more a specific headphone problem.

What kind of fatigue are we talking about here? Is the clamp squeezing your head too much or neck /head becoming sore or are your ears themselves feeling strained?

Beyers are famous for a lot of treble energy, which depending on your age and sensitivity could be fatiguing your ears prematurely, fortunately that's the easiest to fix (and free). You just need to apply some EQ, check out a guide on YouTube for peace equalisation. And check out oratory1990 for his eq settings for the 770 to start from.

Also what volumes are you listening at? If you dont know I HIGHLY recommend getting a cheap decibel meter from Amazon or something similar to check, will be like $20 but worth it to save your hearing from tinnitus in the long run... trust me :(