If you hear it as a flaw, that's what matters. If EQ'ing it down makes music sound better, then why not do it?

While it's true that is where the resonance peak is due to most couplers and insertion depth when measuring, there very much could be a more solid peak there as well. So if you hear it and it's a problem area, definitely feel free to EQ that down

Do whatever works best for your individual tastes and ears.

The best way I have found is play your favorite song, metal tends to work great because it's similar to white noise. Play it with and without EQ. Tweak the settings until it sounds good or at least "right" to you. All ear canals are different especially in the upper treble.

The location and magnitude of that peak can potentially change each time you insert your IEMs, as well as over time while worn if they start slipping out due to ear canal movements, sweat, or wax. What sounds right to you now after EQ might sound off tomorrow or even 2 hours later. Also, good gear is generally tuned with that resonance in mind, which can cause simply chopping off parts of that peak without making other changes to compensate to throw off the treble balance and hurt timbre or imaging.

If you're not sure what you're doing, unless there's either some nasty vocal sibilance (harsh S and T consonants) or an unpleasant piercing or metallic quality to percussion instruments like snares or cymbals that makes them painful to listen to, it would be best to leave that peak alone.

Even if so, I'd suggest trying out different ear tips first to change the depth the IEMs tend to end up seated at in your canals, although do be warned that nozzle size will lead some tips to be incompatible with your IEMs. Foam or Ostry tuning tips (often listed as "turning tips") also used to be commonly recommended for attenuating excess upper treble on chi-fi.

I still don’t understand how to hear peaks in a sine sweep. It just sounds like a rising tone to me.

If that rising tone suddenly gets louder and then softer again, that's a peak. In general, it's easier to hear peaks than troughs. Tuning by ear isn't going to give you extreme accuracy (and you shouldn't try to EQ everything to the same level anyways), but it can help identify big peaks or valleys in the response.

Use a tone generator to figure out the exact frequency at which this occurs for you (I'm sure you know this). Start with a Q value of 5 and do sweeps across that region. If the adjacent frequencies sound too quiet relative to the resonant peak then try a higher Q value. If they are too loud then try a lower Q value. Do the best you can to smooth it out.

if it makes you feel better, yes you should.

Do you like the 8k peak? Since ur not making music and this thread is just about getting stuff to sound good to you, it’s subjective and you can do whatever you want.