The thing to be wary of with D2D is that if you don't make more than $100 in book sales per year they will charge you a fee of $12, and considering the majority of "sales" I get from them are through Hoopla and bring in about 30c, that can be a challenge! Of course, if I didn't publish through the major outlets separately this would have been more achievable, but I'm moving away from them because of this new policy. It's shitty to try and combat the increasing flood of AI books on the platform by punishing small authors, especially when it takes much longer for a human to build up a catalogue than AI.
If you have the tolerance to manage multiple outlets, I would recommend publishing in this order:
buy your own ISBN independently. You can buy through other services but that locks you into them. Buy it yourself!
first through Amazon, both physically and ebook but with expanded distribution turned OFF. This is important because they can fuck up the ISBN stuff otherwise
then through IngramSpark for physical copies, which allows any bookstore in the world to sell them. They make you choose a discount rate between 40-60% for bookstores and recommend the higher rate but unless you're marketing to bookstores outside your local area, just go with the 40%. It already costs us so much more to print than trad publishers, it's the only way we can compete
I also get my author copies through IS because in my experience they're higher quality than Amazon author copies (nicer paper, the spines are always properly aligned). It is however cheaper to get them from Amazon, so make whatever choice works for you
I also sell through my website, which is a combo of Wordpress.org and Woocommerce. .ORG allows plugins which allows you to sell, Wordpress.COM is not the one you want. There's other stuff involved in this, so if you have less tech skills then you might want to use Squarespace/whatever or just not bother, HOWEVER having an author website is a great idea so that people can find information about you in your own words and contact you
Then with ebooks, publish through Kobo (they also distribute to Overdrive), Google and Apple. I'm still looking into Hoopla so I can't advise there, but even though D2D distributes to like 20 different places, they don't seem to be places people use
The reason you would make separate accounts in all these places is because each of them take a percentage of your royalty, and if you use an aggregator like D2D they will also take a percentage of your royalty.
Amazon especially in terms of their physical books is worth doing separately because they cost less to print and ship faster. The quality is slightly worse than IS but it really is a slight difference.
If you don't have the tolerance to manage all those sites individually (and really it's just the set up part that's time consuming and it's entering in the same info every time, so I think it's worth it), I would advise just doing everything through IngramSpark because that'll also get you into bookstores.
You can of course go exclusive with Amazon ebooks and this will get you access to Kindle Unlimited. Honestly, that might be the most profitable way to go, but it does mean that you're cutting off anyone who might want to access your ebook who doesn't have a Kindle, which I don't like. I personally think people should be able to opt out of using Amazon. I also would like to opt out of Amazon, but by far the majority of my sales come through them, so..........
Also, a lot of authors report that unless you're quite popular, the pages read in Kindle Unlimited doesn't really add up to the same kind of royalties you can get through the other platforms, so it's kind of a wash on that front.
Amazon physical books also won't distribute to other bookstores if you want to just do everything through them.
If I had my time over again, I would put a much greater emphasis on getting reviews both from advance reader copies (ARCs) and publications. A lot of the more reputable publications that allow review submissions require the book to not yet be released. It often costs money for those, but having professional reviews makes a big difference. ARCs you should never have to pay for, and ebooks are fine for most readers, but some people go the extra mile and send physical books. I don't know that it's worth it tbh.
You will get people messaging you on social media to offer you advertising in exchange for money. These are scams. They have bought a lot of followers, none of whom are going to buy your book because the account invariably advertises all books regardless of genre and most of the followers are probably bots.
Entering competitions is a really good idea if you're confident in the quality and appeal of your book, but can get expensive, so it might be worth holding off to see what the initial reviews look like before you try your hand. Trustworthy competitions will be transparent about who is judging the books and what criteria they use. The Self-Publishing Advice Centre has a great page that rates the trustworthiness of various prizes (here).