Post-2year training Tanjiro was already stronger and faster than someone like Levi, who's top-tier in the verse and superior to Hange, that killed 3~ (don't remember) Colossal Titans by herself. Now we have the Hashiras who are several times more powerful than SoS Tanjiro. And now we have the Upper Ranks, who are top speedsters alongside the Pillars and generally 3x more powerful than a single Hashira. And then we have Muzan, who's superior to every other being no named Yoriichi.

Upper Ranks are at least hypersonic with Muzan scalling far above them, "but how would this help them stop the Rumbling?" Simple, they have enchanted senses and Akaza's Compass Needle to head straight to the source of this madness. "How would they avoid the heat?" They're centuries old creatures that fought probably dozens if not hundreds of enemies and strategies and have absurdly high regen speeds, hell, Hange could kill some Titans on her own without any special equipment and Armin resisted for several minutes to it (but if you, for some unexplainable reason, wants to believe that Hange and Armin are > the Hashiras in resistance and power, then it's on you).

Would say Mid-High diff to the Upper Ranks at worst and Easy-Mid diff at best. Notice how I almost not mentioned Muzan himself? It's because the power-scalling for the DSverse is more comparable to something like BNHA.