All the story missions and side missions have endings. There's around 20ish hours of story content. Once you've exhausted all of those, then it's up to you if you want to keep playing end-game content which is, as mentioned, repetitive grindy stuff for better gear / character stats.

It depends on your expectations and your experience with Fallout up to this point.

If you're expecting an experience as deep and wide as Fallout 4, no.

If you enjoy a shorter game, but one that has grindy end-game content (like, if you enjoy grinding for loot / gear), you might get a lot of mileage out of it.

Story content is short, and it's not bad. To really "get your money's worth" out of it, you'll want to be the type of player who likes to do dailies, or repetitive "raid" type content.

You're not wrong about one thing: you know what you like. And that's what you should be using, things that you prefer the sound of.

Great "audiophile" (I actually hate using this term) headphones aren't necessarily the best fit for gaming. Gaming audio seems to be work best when the EQ curve is distorted to ridiculous levels, highlighting and exaggerating particular frequencies that give you in-game advantages, but that would sound awful to someone used to "non-gaming" headphones when listening to music.

The DT770 is known to be a little on the lean and dry side. That takes some getting used to. Your brain has to be adjust (and this takes time) from the overly sweetened / salty presentation of a gaming headset. Once you get over that adjustment period, you might grow to appreciate the subtlety, which will help you notice all sorts of little details that might have been hidden by the way a gaming headset is tuned.

Not really, no. Just try your best to listen at the lowest volume that's still usable, and then adjust EQ to suit your tastes.

I'm sensitive to high frequencies, and if you are too, you may want to tone things down up on the higher end of the frequency spectrum. The ideal solution here is to get a pair of headphones that suits your listening preferences as closely as possible without any harshness so that you don't have to EQ. Some headphones are just shrill and harsh, and you end up having to EQ them so fiercely to make them usable that it ruins the overall presentation of the headphone.

Depends on the case. If it has just one case fan, then there's poor airflow / air intake and exhaust; a fan sitting outside and blowing on it won't do much.

Think of it this way. If your room's hot, putting a fan in the hallway won't do much for you. So too with the GPU sitting inside a case with bad ventilation.