Trimming just seemingly redundant words does not help learners improve.¶
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Trimming just seemingly redundant words is not helpful for learners to improve.¶
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Trimming just seemingly redundant words is not for learners to improve, but it is for (describe something else)¶
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Trimming just seemingly redundant words is not for learners to improve, it is, however, for (describe something else)¶

I've given three ways you could have written this sentence. This is because using the phrase "is not for" in English often needs you to have to restate what it is for then after that. This can be seen in the third sentence I've given. So when you don't continue, it makes it feel like what you wrote is incomplete and needs more explanation.

Therefore, you could either:
- Remove the phrase "is not for" and try to use other words to explain your point such as saying "is not helpful", or "does not help" etc.
- Use the phrase "is not for" but add on a dependent clause to explain what it is for. The connective you should use should be negative like "but" and "however". If you're using "however" though, put "it is" in the beginning then however and add in the appropriate commas to make it sound more natural.

Most of the time the phrase "is not for" is used for rhetoric and convincing others so it's a good idea to put it in the beginning! However, I'm unsure how you'll be able to say what trimming redundant words is for so I think

Uand understanding the thought behind it or what they try to say and why they describe it like so etc.

This book is based on corpus analysis, and consists of comparatively highly frequent words and their colocations.

A connective was needed here

My initial target when I started learning English was reading a thick book about religion whichthat I liked without looking up a word in a dictionary, but I fhavore barely gotten to do it successfully.¶
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My initial target when I started learning English was reading a thick book about religion that I liked without looking up a word in a dictionary, Bbut I have largely gotten to managebarely managed to do it successfully.¶
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My initial target when I started learning English was reading a thick book about religion that I liked without looking up a word in a dictionary and I have managed/gotten to do so successfully.

The word "which" is used for specific details, however, the detail that you like this book regarding religion is important in this sentence, so "that" should be used

The word "favor" is a good one and it can be used when you want to say that you prefer something over something else, let's use your sentence as an example: "My initial target when I started English was reading a thick book about religion, which I favored over others in the genre," However, this wouldn't be appropriate here since you don't want to say you like the book more than other books on religion, but you just want to say that you like it/you're interested in it in the sentence.

Commas are quite a tricky thing and to explain what I did would lead me to write a whole essay. A good article by Grammarly summarizes it well: https://www.grammarly.com/blog/how-to-use-commas-in-your-writing/

Then I also became also accustomed to reading random stuff in news, especially from one news publisher online based in Japan.

The word "also" is put before verbs in sentences, not after.

The next target is to read rapidly on the news as soon as possible on news, especially focused, focusing especially on those about individual stocks.¶
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The next target is to rapidly read the news as soon as possible, focusing especially on those about individual stocks.

And I also have much of a chance of output using English thanks to the development of virtual society and borderless one, including this site we are in.

I am certain that this dictionary will help me to be armed with more accuracy and naturality close to the native speakers.

Ultimately, I could go beyond them at least in some parts if I keep my studying constantly and strenuously!