I guess this is my answer then, the Dolphin is just a friggen superhero. I can charge FSD in the Dolphin while literally AS CLOSE AS POSSIBLE to the star (without dropping out) and it never goes above 65%.
If this isn't possible with the DBX through engineering or any similar such thing, then I'm selling my DBX. I'm not interested in having the risk of temperature damage while doing cartography in deep space. Even if I can change my method, it'll slow down an already slow process while making a tedious journey even more tedious.
Your linking from coriolis was fine. As for the build, my explorer has some heat reduction tweaks, most of which have already been covered, but other than that yours is fine - I guess your expectations have just been framed with the Dolphin as your reference point, which is a bit like asking why a Cobra Mk3 has such a long time to kill when you're comparing it to an FDL
I'll tell you one thing you could try which has a minimal effect on your mass and therefore jump range, doesn't cost much more and does make a difference. Swap your powerplant to a 3A and then engineer it with grade 5 efficient and thermal spread experimental. Has a big effect on thermal efficiency.
Other than that, I hate to say it but try an Asp or go back to the Dolphin. The reason I hardly use my DBX is that scooping takes too long for me with a class 4 scoop and if you aren't scooping for as long, you aren't heating up so much either.