Hello, congrats on your success.
Thanks
I am in a low competition niche also and I found that if my site is niche-targeted and I do on-page SEO properly, it will climb in the SERPS. It's slow tho. But it surely does climb. At this point, I consider on-page SEO as an art really.
How long does it take you to start seeing traffic?
It varies. At first, my aim is not to really "grow traffic" (it's more like a nice bonus) as much as it is to "create" the foundational (supportive) content for the site itself.
What is this my "foundational content"? Let's say the product is fish tanks. Before I can "review" fish tanks or make "best fish tanks" pillar post or the like, I first need to start with the most basic shit there is. What's a fish tank? What's a fish tank made of? How to clean it? What type of fish can I put there? Where do I get the water? How do I make sure the fish don't die there? etc, etc, etc, etc, etc, etc, etc - "fish tanks" is a humongous niche to cover and would maybe literally take 300 info posts to cover just barely. Actually, it's a bad example because the niche is too big to define who the target audience is. I just don't know much of anything about fish tanks. Maybe a fish tank for shrimp is different from a fish tank meant for gold fish, so it's like comparing apples to oranges. But I personally want to know where my target audience hangs around, are they mainly women or men, family people, business setting instead, etc, because without knowing this stuff it'd be like using a shotgun to hit the tiny mark 250 meters away (can't exactly see the mark clearly, won't even exactly know if you hit it or not).
Anyway, that's the foundational content that I consider just "filler content" that supports the type of content I ultimately want to rank (the "best niche product" type posts). The reasoning is, if I've covered everything related then Google must see that my claims are not baseless and the reader can look up everything to squash their doubts about fish tanks for their particular use. Sometimes I can get away with less supportive content than I originally planned because the competition is just too weak to stand a chance even if I half-assed it leaving me with some leeway in the future in case they started upping their game to stay relevant.
So then, it takes exactly as long as it takes for me to cover the niche with the planned content.

The P2 is the main affiliate site,
P3 is an experiment with cheap content,
and P4 along with additional projects are trying to be new P2-equivalents but for which I have only partially finished the foundational content.
Do you use category to silo your content or page -> sub-page?
Well, yes, of course; categories look something like this: 1) reviews, 2) info posts, 3) extra (e.g accessories)
It's a simple means of navigation. Can't do without. Could be done better, but I like to keep things super simple and easy.
What is the strongest on your site?
Pillar money posts, the main posts, the type of posts everything else revolves around. ("Best" type money posts)
Everything else, as has been mentioned many times over, is supportive content (the reviews, info posts, etc).
What do you put in your homepage?
Just posts.
Hard to find decent themes nowadays. Every1 is copying whatever the current trend is, and they all suck. Incredible amount of bloat. I can't even find an example for this very reason.
Is your main money maker site a niche-targeted site?
Yes. It's because it takes less effort to get on top of a small hyper-focused niche. Bigger niches would require way, way more work with this approach.
With internal links anchor text from info content to pillar content, do you maintain a proper anchor ratio or you just use what seems natural? Do you just do a sentence type anchor or generic anchor?
Whatever makes sense.
And it's mostly the other way around - the pillar article links out to all the supportive information. It would hardly ever look natural if you linked to a sales page from info posts. But you do you want a link pointing towards the main money posts, so you just have to be extra crafty to come up with these links on top of the normal interlinks that genuinely make sense.
Hope it helps.