Finally rainbowed Naga City:

I went straight caster (a.k.a. the "Gnome Sorcerer of Mystera With Horns"), prepped Mage Plate and got a lucky Dragon Soul, which - combined with a rather perfect Glyph draw including PISORF and BLUDTUPOWA - was sufficient to plow through the bottom part. I had no food issue thanks to the Monster Den secret sub (benefit #2 of PISORF was that I could access it).
The key to success was that I finally learned how to manage the conversions properly - to fully leverage the restores, alternating between converting glyphs without reaching the threshold, and then converting something else to reach it. The best way to descend is just shy of the conversion threshold, with all glyphs in inventory (hence the compression seal); so popping some food gets the first full restore, and then the first glyph will almost reach the next. Health Potions are useless, the ones found topside should be used up for bonus XP, and the gauntlet's Keg of Health is perfect conversion fodder. The Amulet of Yendor is also best used as conversion fodder, it nets 100 cp and the XP bonus is nigh meaningless for the goat.
This strategy has some moving parts, i.e. it requires some boost, in the form of finding Crystal Ball (best) / Battlemage Ring (good) or Dragon Soul (so-so), but prepping Quest Items means that most of the time something useful will spawn. I chose to prep the Mage Plate over the Crystal Ball for the early-mid game mana bonus, which is arguably suboptimal versus other items, however it's still a little bonus that helps round up Mana to 18 or 20, and the Mage Plate itself is top notch for the late game.
"Thinker", just without the "ache".