So, if I've understood your situation, you can exchange your pair of phones for simething else at Hifiman and 130$ will be knocked off the price of that something new?
And the 200$ limit you have is the total, which would mean the most you would do is another 70-ish$ above the 130$?
If that is the case, and looking at what measurements are available, the 400SE seems fine.
That being said, if your pair are not defective, you may not be getting an upgrade.
If you're willing to add more money nothing under the Sundara makes sense.
You could always sell what you have (I've read your concerns, it really is not a lot of money, I've sold much more expensive stuff over ebay). If it sells for, say 100$ then you can add another 100$ and have more flexibility.
As people have mentioned, the 6xx becomes an option but personally I'd recommend the HD560S. I have them and the Sundaras, they have better bass extension and measure really neutral, compete very well with the Sundara at 200$.
Someone already mentioned, don't fall into the rabbit hole. You've used some of the language: "elite reviewer" for example, which hails the start of audiophoolishness - but hopefully coming to this forum will nip that in the bud. Atom stack + the 560S is affordable neutral clean sound and you can always EQ to change the sound if you prefer something different since the foundation is clean.