LOL, you have played more Bach than me. I have played one invention (#1), one sinfonia (#15), one WTC set (b-flat minor, book 1), and one other WTC prelude (c major, book 1), and two partitas (2 and 6). I have played around with some other things, though, and partita 4 is one I have spent a bit of time with.
I think I know what you mean about finding certain movements boring, because I often find myself in the same position with Bach. Sometimes his keyboard technique is particularly counterintuitive, and sometimes it's hard to get your hands on a good interpretation. Anyway, I have learned not to trust these intuitions with Bach. It's always good. Sometimes, the music requires a little deeper digging. I just started working on partita 2 recently, and the courante was the movement that I had trouble getting into. In partita 6, it was the gavotte. In partita 4, it's probably the allemande. Allemandes always seem a little harder to get into than other movements in the dance suites; they're not quite as profound and angsty as sarabandes (like that lovely one from English suite 2 that you liked), and they're not as catchy as the faster movements. I partita 4, I am quite fond of the courante. I will probably never be able to play it as fast as GG though, and that makes me sad. The minuet/passapied are also a lot of fun I think. Sometimes, the movements that are the hardest to get into are the ones I end up liking the most.
A good tip if you are bored with one of the Bach dances: spend less time on the A section than on the B section (unless it's one of the rare non-binary ones). The real meat is always in the B section, and often the best bits of the A besides. Play hands separate, and voices separate as well. Play hands together, bringing out a different voice every time. And above all, I think it is helpful to think of your fingers actually dancing the notes of the dances, instead of just playing them. It makes it more fun to me, as opposed to just being work, and it also helps me keep out tension.
I think that Bach can probably be a bit like a miracle drug for piano technique, but I think you have to get into it for it to work. I have always liked Bach, but I have always had a hard time playing most of his music, so I stayed away from it. I think a lot of pianists in the last 200 years have probably felt the same, because after the piano took over the harpsichord, no one wrote like that any more. Keyboard technique changed completely, in order to take advantage of the full range and percussive/expressive potential of the instrument. Mozart was conservative on those terms; Beethoven tried to orchestrate for piano; Chopin perfected the art of pianism, and his contemporaries weren't bad at it either; Chopin has a lot to do with why modern pianism is so different from Bach's keyboard technique, but at the same time, you can see how much of Chopin's technique was inspired by Bach's.
But, most kids prefer the easier Chopin things, the easier Schumann and Liszt and Debussy and Rachmaninoff and maybe Mozart and Beethoven too, and generally stuff where you only have to lift the damper pedal every now and then, and it's all legato and essentially homophonic. And it's not that those composers are actually easier than Bach; they all wrote a good number of things that are fairly difficult that most kids would simply rather play...because they're flashier? Bach is sometimes flashy. Prettier? Sarabandes are about as rip-your-heart-out as they come, but they are harder to pull off on a piano, for sure. It's hard to describe, but Bach was old-fashioned even in his time, and he's certainly considered old-fashioned now, so old that he didn't actually write for our instrument!
But, as a musician, I find his music the most rewarding to learn. Chopin is not far behind him for me, and sometimes I think I only put Chopin behind him because the stuff I want to play is so difficult. But the more Bach I play, the easier it gets to play Chopin, and since I suck at piano, I have just been playing the same Bach over and over again, and therefore haven't played much.
So, to sum up:
Is it possible for me with such a poor experience with Bach to learn Bach alone?
With which piece schould I start?
One that you like, and one that challenges you. A suite would be good, cause it will give you a wide range of music and technical problems.
Which practice-methode is recommendable to benefit from Bach technically?
Make it fun for you. If it is boring, then spice up the articulation, suspend notes that aren't written that way, do crazy ornaments on the repeats. Bach is really flexible.