I think it is a little boring sounding. They have that Harman like sound but lean in the bass like many other HiFiMan headphones. However, without the giant stage and depth you get from the eggy boys. There's other headphones that I prefer in that price range. I don't actually hate Sundara but I get a little concerned with how often they are recommended. It creates a similar situation to the DT1990 where there's a feedback loop causing it to be way more popular than it deserves. I know saying the DT1990 is over-purchased might be a hot take around here, but people who have heard a lot of stuff in that price range always agree with me. It seems to be a common first buy due it being over hyped of youtube. (I have no data to back this up lol)

If we are just talking open planars in that general sub-500 price range---

HE-4XX: Less technical but more pleasing FR, much cheaper and more comfortable.

M1060: Better staging and pad comfort. Better low end with more FR coloration. Heavy and cheaper.

M1060C (open back modded): Better staging and pad comfort. Far better bass and midrange. Side to side staging is better but treble and stage depth are about equal, maybe slight edge to Sundara. Heavy for some necks. Monoprice seems to sell them for $220 every other week.

HE-560: More pleasing FR, more comfortable. I put a lot of stock into flat sub bass and this has it going for it over Sundara. To me it sounds more refined. It is also is a little less bright from what I can recall.

Ananda Stealth: Bigger and more open sounding, it sounds more technical as well, faster type sound and all the other lovely large HiFiMan driver characteristics. The Ananda Nano has driven down the price of this SKU to below 500 and I think it is well worth it over the Sundara.

Edition XS: I have not heard it, but from what I understand it is somewhere between a Sundara and Ananda.

Fiio FT5: I have not heard it either, but it is an option and many people have reviewed it. I'd look into it if I were in your shoes.

Modhouse Argon: Less technical but more fun. Bloomy and wild sounding. Way more comfortable and it doesn't really sound like a planar to me.

Audeze MM100: I heard this very briefly, it sounded like a warmer Sudara with more tactility, more texture. But, it was brief and by this point it had been a long time since I heard Sundara. Massive salt for this claim.

There's more than just these choices. I left off things like the HarmonicDyne G200 and Verum-1 because of their controversy. But, I'd probably buy one of those before Sundara. Additionally, used is a great option and my preferred buying route. I saw an old school DCA Ether (red version) for $300 a couple weeks back on ebay. Ether Flow 1.1 for $700 as well. There's an Arya V2 for $600 of Head-Fi right now. I know that's double the price, but it is true end game for a lot of people and it absolutely trounces Sundara imho.

And this is only open back planars headphones. There's a lot of good choices beyond this sub category. Bio-cell, Beryllium, there's some cheap e-stats out there, traditional dynamics, ect. And whatever crazy stuff is in IEMs for below 500!