Unfortunately I am not familiar with US prices because I am from Europe. But if it is a discount and tax not included, $250 does not sound that crazy cheap, so I would not guess that it's a scam judging from the price alone. But you should still make sure you're not buying from a scammer, I cannot guarantee you anything. But if it's directly from Amazon / from a reputable seller / or if you have buyer protection, you should be fine.

Yes, exactly that one.

But if you can opt for an open-back (that is if you don't want to use it outside or indoors next to other people), I would definitely recommend the R70x over the TH610. It will most probably sound way better and be way more comfortable (although that of course is a matter of taste). And it costs only half of the TH610.

I'm sorry but I don't think there is something for 300 bucks that will be a substiantial upgrade. The M40x you already have is a good headphone. What exactly do you want to improve? Do what you want, but we all sometimes get that upgrade itch although we have a perfectly good headphone on our head. My suggestion would be to cough up some more bucks and buy something like the Fostex TH610, which will be a substiantial upgrade.

That's provided you want another closed-back headphone. If you want to try an open-back, the ATH-R70x will be a huge upgrade. It trades blows with headphones around the kilobuck territory.

Not really, but kind of. DT 770 was my first headphone and I obviously liked it (fat sub-bass and spicy treble). With the knowledge I have today I would not buy it again. But can I really call it a regret? It's sturdy as hell, easy replacable and cheap parts and can be EQ'd. Then, I am still going down the IEM rabbit hole and have a bunch of IEMs that are not outright bad, but none of them satisfies me as an endgame should.