There are a few things I would try which I call 'stupid-simple' because they don't make sense, but might work

Always keep in mind that everything is connected, and equally important. Those that focus only on ads are making a huge mistake.
#1 Interests
First, targeting interests in the US is completely different than the rest of the world. Unfortunately, gathering that data was never important enough for FB, especially outside the US and EU and the EU has too many restrictions that it would be worth for them to do it.
So you will have to use common sense while targeting interests, and know you are restricting the audience, and leaving a lot of people outside. In your case, I wouldn't focus on just a few interests and would research and check if the big bookstores, publishing companies, authors, anything that would interest someone in your niche could be targeted.
Audience Insights won't give you correct numbers but will give you an idea of who the audience is, what gender and age so you could avoid right from the start to target everyone. You can target everyone once you have a bigger budget and can leave the ads running for a few days, but definitely not with $5/day.
The most important thing that the majority don't know or understand is that
every time you create a campaign you hit a certain pool of people, and will NEVER get everyone. That's why it doesn't usually work when you analyze the data and create a new ad set with age/gender that worked in another ad set. Again, every time you publish an ad set you will hit a certain pool of people. Will there be people interested in your offer? Maybe, maybe not. Maybe there are a ton of other advertisers that day that will get a higher quality audience, or show them better ads, with better offer/product, etc.
A simple thing that I always test and sounds a little bit stupid is that I duplicate the ad set that worked 3-5 times, change the age/gender/placement (whatever that worked), and run them on a small budget and keep an eye on them through the day. Once they spend a certain amount of budget I analyze CTR, ATC, IC, ... and act accordingly. At least one of them will perform because all of them are most likely targeting different people. I know this sounds weird, but it works 90% of the time
#2 Budget
You know that $5/day is nothing in ecom, especially targeting such a broad audience. This budget might work with a smaller audience. Try an ad set where you use 'Must also match' option and stack up the best possible interests and see what happens. Again, this might not work outside of the US.
I understand that the majority can't afford bigger budgets at the beginning, but you must understand that this is not how you will be able to compete with others and make this successful. In order to spend more, you have to make more.
Those that can spend more to acquire a customer win! So how do you do it?
Besides focusing on improving your ads, testing, etc. you have to invest time to improve your conversion rate and average order value
ALL THE TIME. I can't stress this enough. Small tweaks can bring you at least 20% higher AOV which will help you with testing more ads, pushing more traffic, making more money, and repeat again
To get high CR you first must send the right traffic, have an appealing product page and description, your site must be trustworthy, think about free shipping, etc. For higher AOV create product bundles (more of the same product with discount), 2-3 related product bundles that they can't find anywhere else, use post-purchase upsells/downsells and always keep in mind that just because you are done selling that doesn't mean they are done buying
#3 Retargeting
Most people don’t buy on the first visit. To boost your sales, you have to set up campaigns and remind them about your offer. 90% of the time retargeting campaigns will bring you the highest ROAS, better CTR, lower CPC, and of course, low CPA.
Customers came to the site for different reasons, and they visit different products/categories. The pages they visit represent different interests.
If you set up a simple retargeting campaign on Facebook, Google, you will cover 80-90% of the internet. Don't overthink. Start small and build up things; you don't have to be everywhere right from the start.
A few important things:
- Don't include everyone in your campaigns. Create different audiences and retarget them based on where in the buying process they are.
- Frequency. This is important. You don't want people to get annoyed by seeing your ads multiple times per day and think you are desperate.
Best audiences to retarget:
- All-time visitors or visitors of a specific page (Non-buyers)
- Anyone who engaged with your Facebook page/IG/watched a video
- People who abandoned cart
- Your email list/customer file
- Existing customers (Buyers)
Two must-have retargeting campaigns that can be implemented very quickly (low-hanging fruits ready to be collected):
- Facebook Dynamic Ads - to remind them of what they visited. In order to run this, you need to set up a product catalog first. Shopify/Wocommerce has plugins that can help with that. To improve conversions of your bestsellers, create a custom image/video.
- Google Dynamic Remarketing - similar to FB Dynamic ads. Include images, and bid more on visitors that are further in the funnel (like ATC).
#3 Lookalike audiences
Lookalike Audiences are performing very well lately, especially in the competitive markets. Always test as many LAA's as you can and stack them to improve targeting or use them to narrow down broad audience.
Below is a list of Lookalike audiences that can improve your targeting:
- LAA of higher spenders → import a CSV of top spenders OR use an LTV option directly under custom audience, and create an LAA.
- LAA of Buyers, IC, ATC, website visitors
- LAA of email subscribers → import CSV file and create LAA.
- LAA of people who spend time on your site → create an audience based on Visitors By Time Spent/Top 25% and create an LAA.
- LAA of people who watched at least 75% video.
- LAA of everyone who engaged with your post or ad in the last year.
- LAA of Facebook fans.
I hope this helps!