What was the age of Fuse when he was writing Tensei Shitara Slime Datta Ken(WN)?

What I've found that the problems you see in this book are fairly common in Japanese web novels.  The problem with them is that they tend to be written by complete amateurs with no experience in creative writing.  All they can do is draw on the material they read, and a lot of the material they read is just like this.  

The way I see it is that a lot of this kind of writing would be okay in a manga where you have other visual detail to add to the story, but it sucks when all you have to work with is the written word.  For examples, a lot of the expository material is taken care of by just seeing what the action looks like, and the first person problem is a non issue because the POV is handled by not being in your face.  The fact that many protagonists are purposefully designed to be nonentities doesn't help things.

There are better Japanese web novels out there.  Usually this happens when the writer can bring something extra to the story.  But as this tends to require either real life experiences or that the writer has interests outside of otaku material, it's not all that common to encounter.  I've found that Chinese web novelists tend to be much better when it comes to the mechanics of writing, but bear in mind that I don't read xianxia novels and these tend to draw the biggest complaints. 

That’s not slavery, that’s just desperation for attention and validation like wattpad. Except for a few who just want to experiment. Anyone of them could just put it up on Amazon instead if they wanted to. 

What irked me was from watching the anime.

All that, "Oh shit, he's attacking me. I should try to keep talking instead of beating him with my power". Never really paid attention to how much he talks while reading the novel.

Mind you, I mostly read CN novels. 

That's a basic "bait"-style to get the readers to want more, or to tease the readers. The Author can later explained them in future chapters, or in spin-off, or in some different series related to the original novel, or in some cases even forget about them.
And who know, did Fuse ever introduce himself?
If you are frustrated by that kind of writing, you aren't even a "common reader", but a "newbie reader", since it's one of the Most Common writing, and mostly used for Foreshadowing or stuffs. 

what is this?
do you mean that things get thrown into the story without getting explained? 

I got bored reading slime novel due to lack of thrill on it and I read ReMonster lol. The world building is a bit lacking plus everything seems to fall on his lap. There is no intense struggle that can exite me just like other isekies. The foreshadowing is lacking of punch that it is not supprising.

Age don't matter much but experience, luck, talent and mentality.