Is this your first self-publishing project? Assuming it is, I say, “Keep it simple!” Lucky for us, we can start at the top, as it were, by starting our self-publishing journey in Amazon-land, using Amazon Kindle Direct Publishing (KDP). Why lucky? Because Amazon already sells more than 70% of all e-books and can give you access to just about any market or reader audience segment. So, the tool/platform I recommend, especially for starters, is Kindle Direct Publishing.
This doesn’t really answer the question, though, does it? As you already have seen, there are dozens, at least, of tools, platforms, distribution channels, and publishing services providers to choose from. I’m just saying if the leading provider of just about everything is free/cheap and easy to use and you get all its massive marketing machine working for you, why not start there? There’s plenty of time to get deeper into the marketplace and engineer the perfect set of capabilities and resources for you. Whatever you decide, best of luck to ya’!