Try the regular 3.5mm cable, rotate the cables around a bit and see if it's properly plugged in. Try them with your phone/other devices or directly (without using the FiiO K3/anything else) into your front/back panel 3.5mm ports/plugs for the computer.

Make sure your glasses/hair/piercings/etc. are also not in the way, they may be affecting the sound.

For comfort with headphones, if you have a big head, ear piercings, glasses, et cetera, basically anything that will interfere with the seal/fit of the earpads, here's more info about headphones and glasses and workarounds it: https://www.reddit.com/r/HeadphoneAdvice/comments/zkjjcu/are_there_any_headsetsphones_that_are/j00kjt2/

Essentially try to isolate if it's a headphone problem as HiFiMan is infamous for quality control issues.

See here for a bit more info on HiFiMan quality control problems: https://www.reddit.com/r/HeadphoneAdvice/comments/zicl2e/most_common_defects_for_the_he400se/izrcf3p/

Otherwise yes it's probably just the expectations. Try IEMs instead as they are often better for value.

If you already tried parametric EQ, still go on squig.link and see if other sound signatures of headphones are to your liking, it's not gonna be like the other headphones but it should give you an idea.

See here for more info about Qudelix 5K, parametric EQ, squig.link comparison graphs, AutoEQ, et cetera: https://www.reddit.com/r/headphones/comments/109meb5/eq/j40cy1n/

Look up oratory1990 (check the subreddit, /r/oratory1990, don't forget the preset parametric EQ list (mainly for headphones but there's some IEMs there): https://www.reddit.com/r/oratory1990/wiki/index/list_of_presets) and crinacle's articles on what amps/DACs/etc. are and what they do.

More info on headphones/IEMs and PC gaming: https://www.reddit.com/r/HeadphoneAdvice/comments/1151ezm/looking_for_1st_real_pair_of_headphones_mostly/j8zm0q8/ and https://www.reddit.com/r/headphones/comments/10zxpfb/why_are_you_using_openback_headphones_for_gaming/j85vv18/ and https://www.reddit.com/r/HeadphoneAdvice/comments/10ks4tr/looking_for_iem_for_gaming_and_music_listening/j5sjhyh/

Open back headphones noise reduction (scroll to the bottom half part of the comment), earwax clogging issue, parametric EQ, noise reduction for your PC/room, et cetera: https://www.reddit.com/r/HeadphoneAdvice/comments/115c99b/coming_from_iems_and_looking_for_headphones_150_eu/j92mpe9/

I used the HE400SE with the FiiO UTWS3/5 TWS adapters and they were just fine too. Again just make sure HiFiMan quality control didn't strike you and it should be just fine.

And yes don't forget the library or genres variability part, for some people they need a bit more bass or say less treble with some headphones when it comes to certain songs/albums from genres and so try different songs from your usual library and compare it to your friend's Sundara/other audio stuff.

The other main recommended headphone in that price range is the Sennheiser HD 560S.

These days Sennheiser is always running them sales on their refurbished stock of the HD 560S/etc. and these are sometimes the new revised version (different cable/etc.), btw, but they are significantly cheaper. I've heard people are also able to get another (replacement) unit if their package is like clearly used/dirty/etc. but it's normal for the dust/etc. to be there though. Check /r/buildapcsales for those refurbished/renewed/used/etc. sales, it's usually with Best Buy and Amazon too.