I have done the math on the Monk and I'm not sure where you're getting 6 times. Since the resist cap is 75%, that's only 4 times. It's also ignoring the fact that Monk's don't start with 75% resists, so any comparison needs to have the other class with 25% resists. That drops it down to 3 times. Throw in the damage penalty (which makes the monk do about 65% the damage of another class) and you're down to only 2 times.
That's fairly good, but certainly not 6 times. It also ignores the fact that the other classes have other abilities that quickly make that up. For example, the Paladin ends up doing the same damage (thanks to his starting resists), plus has Monotheism and a free HALPMEH, which I prefer over the double regen.
As for the Thief, his power level is deceptive. His extra 33% items gets you one of each power up, two potions, a glyph, and (I think) some gold. That's a pretty massive boost and I think that one ability is more helpful than the entire Fighter class in Naga City. Btw, I find it amusing that I'm arguing he's stronger than you think, while you're arguing that he's a cheat code
Finally, I'd suggest you do try Body Pact. If you think resistances are busted, just wait until you pump every class up to >40% resists while still getting the advantages of another diety (Taurog and Earthmother are the best - TT can work if you're VERY careful about getting hit exactly once by each monster). As you try out Body Pact and get more experience with GG, I suspect you may reevaluate what you consider a cheat code.
But we can at least all agree that Bleaty cheats, right?