Sorry, not a comment on the actually body of the post itself, but the closing; 気をつけて doesn't mean "take care" in the sense of "be well" but "be careful" (as in, "take care not to hurt yourself"); I don't think it makes sense to end posts with it.

Teaching well is extremely hard and takes a very large amount of effort. I think you should evaluate what specific topics you can dedicate the most to if you really want to make something worth reading. You'll need to narrow down from something as broad as web development or Linux development.

On the other hand, you don't need to have the end goal of having useful blog posts. Practicing writing and communication is useful in it's own right to you because they are useful skills that we don't get as much practice as we should. But still, I think it comes down to picking something that you are able to spend a lot of effort on, because if you're not spending a lot of effort you're not getting as much practice as you can.