I had no troubles with the things Twain laments. You can rather quickly learn (but not internalize) the rules, which are exactly the same as in other Indo-European languages. Compound past tense as in Romance languages. Declination like in Slavic, and in Instrumental case the ending is almost the same!

The biggest trouble nowadays with learning German is that all textbooks are DUMB and don't give enough practice. Basically they're tests on steroids. This is not the way you can internalize the grammar. You may know the rules, but still unable to use them correctly when speaking or writing.

All the books are shiny, with lots of drawings, photos, bells and whistles, even media content over an app, etc. But, as said, none of them contains enough excercise to practice grammar. Over last 6 years that I did an effort, I've never seen a textbook step away from this format.

Teaching by Goethe Institut is equally awful: most time you excercise by inserting just one word in a sentence spellt for you. When finally you reach speaking at length and not to the given pattern, everything falls apart, and you're told off: "oh, so many mistakes, go repeat the grammar rules." (No wonder most students choose the strategy to just speak at the kindergarten level. Das Bau ist grün. Berlin ist die Haupstadt Deutschlands. Ich möchte in einem Vorstadthaus leben.)

Germans! Admit, you conspired together to not let us learn your precious language!