OP I'm in love with your journey as you've provided real results and have taken us through your struggles and growth along the way.
I'm just getting myself into low content books and I'm learning so much along the way and it sounds like getting into the different types of niches is key.
I just started getting my low content books up the other day so far everything is still stuck in review but I've posted a few different types of low content books such as 2 notebooks, a journal or diary, 2 recipe books one of them will be a Mother's Day gift, a graduation guest book to get ready for the spring, and I'm working on two gamebooks from PuzzleWiz thanks to the OP for sharing that one. I'm definitely gonna get my head into every niche possible.
I've also done a ton of research and gotten some real help from StackinProfit's YouTube channel, you guys should really check that out if you already haven't he's got some great information in there.
He preaches uploading a lot of content to play the numbers game but also focusing on what genres work for you.
I'm not sure how I'll do yet but I want to make a full-time living out of this so I'm aiming for $1,500 per month minimum from this and I think it's doable but it will be challenging.
StackinProfit seems to have taken advantage of holiday seasons that seems to be the key in generating sales is focusing in on holidays or school supplies season from July-September. StackinProfit's first success came from Valentine's Day notebook that sold really well around the time he started last year. I just missed out on Valentine's Day season because I would've done custom cards as well but I'll try that for Mother's Day.
I think it's all about selling stuff that already sells but getting into niches that aren't too competitive as well. The most saturated niche seems to be notebooks and journals but I'm still going to do them no matter what because people will always need them.
My goal starting out is to see what sells the best for me and start to attack that niche. I'm not going to focus on the number of uploads too much I'm just gonna see what does the best for me and just try to scale that up. I think the most difficult battle is using the right keywords and finding the right category. The categories are difficult for books.
I'm doing everything organically too I don't want to use any paid promotion until the 4th quarter of this year.
I'm still going to try my hand at ebooks as well when I have time for it as well with the short book pamphlet method. I'll do my own journey thread if things go well through April.
I'm branding everything under "Saved By the Books" so if you see that name anywhere on Amazon that's me.

Here's my covers for the recipe books: