I have personal experience which may help you in this endeavor.
First, an editor is extremely important. Even if it’s a fellow grad student with strong grammatical ability, have them read it over and do a favor for them in exchange.
Secondly, most self-published books, even ones written by someone with a great mailing list, are not enough to make a sustainable income. It just isn’t. You’ll likely see a surge of sales at first, but even those may be discounted ones if you choose to do a promo to get more readers to buy it at first. (I did the same thing with having a mailing list opt-in in my book, as well. It’s marginally effective).
You have all this content, yes. And if you’re using the medium partner program, that’s a bit of money.
You say you aren’t happy depending on freelancing, but what type of freelancing are you doing?
If you cultivate monthly clients where you do weekly blogging, you’ll create much longer-lasting relationships with a much more dependable income. Instead of grinding out projects or pieces here and there for many different clients, focus on finding 3–5 stable clients who need long-term services like weekly blogging, email and ad copy, product reviews, etc.
If you have 5 clients and charge each of them $1000 per month, that’s $5000 per month, equivalent to a $60,000 salary — which is what I was making within 3 months of starting freelancing because I focus specifically on monthly clients.
In your contract, set a minimum, such as $1000 per month with a six-month minimum commitment.
Now, because of this strategy, I’ve cultivated some great long-term relationships and ALL of my current clients are referrals from previous ones.
You CAN make a living blogging — and you’ll do it a lot faster by blogging for others.
Find entrepreneurs or small to midsize businesses who don’t currently have or regularly update their blog and reach out to them. I found the majority of my first clients via Instagram (I know, crazy), just by reaching out and messaging them asking if they had thought about having a blog on their website for higher Google search rankings and keeping site visitors on the site longer. You’d be surprised how many people need that assistance.
And I’ve never had a real portfolio — I use my Medium blog as my writing samples.
So, sure, put together a book. But you need more than just editing — you need a nice cover designed and interior book formatting, too. And be careful to manage your expectations, because most books are not moneymakers on their own.
I am not trying to drag you down or talk you out of publishing a book, I truly just thought I might be able to help.