Reedsy’s comprehensive guide to eBook distribution is worth reading if you’re considering self-publishing:

https://blog.reedsy.com/ebook-distribution/

I detailed my own experiences of digital publishing in an old thread:

https://paulpens.cloudaccess.host/digital-publishers-distribution/

One thing that’s not mentioned in Reedsy’s otherwise excellent article is a drawback of ‘going wide’—that is, using an aggregator to distribute your books or doing it yourself, publishing to Apple, Kobo, etc one by one. Should you change your mind, deciding to sign to KDP Select, then Amazon will insist that you take all your titles off whatever sites they’re for sale, before they allow you onto their hallowed ground. In theory, this should happen when you unpublish them on Smashwords or D2D, but in reality, it can take months and many emails to vendors before that happens—meaning your books are in the twilight zone, off most sales venues, but not on KDP Select.

At the moment, I’m contemplating removing my books from D2D to go with KDP Select for my Cornish Detective series, which I think makes commercial sense, but looks like creating loads more frustration for me.

No one said it would be easy (but, why does it have to be so hard?)