Interestingly what I did find out, not only for DMT, but for many substances, that vaping bad tasting or harsh substances is easier, if you add a tiny bit (1mg) of menthol crystal. They are cheaply available in pure form as sauna equipment. This kinda gives you this typical menthol smell and additionally opens up your lungs.
But then again, I don't find DMT to be really harsh.
But what I also did find out it, that the temp of the inhaled air makes a huge difference in harshness (also for DMT). If it is hot, it is harsh (no wonder as very hot air certainly irritates). That's why I now have like a 20cm thin walled metal tube as mouthpiece. I flattened the metal tube a bit to get more surface contact. If you now hold it in your hand, the hand will cool the metal, which gets heated by the air inside.
Additionally if bad (after)taste is what puts you off (e.g. see DPT), then you can put this metal tube as far in your mouth that it is right at the back. If you now inhale, no smoke will enter in your mouth room, as it will directly be sucked down your throat. This has the additional advantage that less material condenses out in your mouth.
If the substance is really bad tasting another addition to the said above works wonders. Put a paper handkerchief on your tongue while vaping, so that the tongue can nowhere get in contact with the smoke.
All these might sound strange, but work for me quite well...
I claim not that this is the truth. As this is just what got manifested into my mind at the current position in time on this physical plane. So please feel not offended by anything I say.