The 8th is a barrage and a tour de force. And it has some amazing moments.
But… IMHO you’re on to something there. It is something of a step backwards for Mahler. It is not less competent, but it is less personal, and musically more conservative than what he did before or after. I love the relentless first movement, which is Mahler’s kind-of Mass, and is loud and energetic enough to match anything else he did in that vein. (Hostem Repellas!) The second movement, which is his kind-of Opera, relies on Goethe for its structure rather than internal logic, and I think it sags.
And some of his aesthetics in this piece has not survived the test of time, where I think most have. The harmonium and the whole Ewig Weibliche thing is too much of the 19th century.