I have said in this space that unless the first book is a best seller, writing sequels is for chumps. It is a well known fact that every book in a series sells fewer copies than the book that proceeded it, as readers drop out for various reasons. Thus, one needs a high initial base line in the first book in order to be able to accept the declining sales of the following books. Nevertheless, you often see writers writing, and selling or self-publishing a trilogy before the sales of the first book is known. And if it fails to sell, the remaining two books are dead on arrival.
I ignore my advice. Of course, I do. None of my books are best sellers, and yet you will find that most of my books are part of one series or another. Three or four depending on how you count them. I wrote the what became the Bright Black Sea with the idea of it being a serial story of novellas back when Amazon was paying Kindle Unlimited books on a per book basis, rather than by the page. I could've released it as three novels, but I didn't want to see that inevitable decline in sales, so I bundled them all up together to avoid that heartbreak. The Lost Star's Sea was similar in size and structure, mostly because I just wanted a companion volume to be of a similar size. All I intended to write was the castaways story, but had to continue to fill the book out with other adventures. I have since broken those two novels into a six book series on Amazon in order to make them audiobooks - they were too long for there auto-narrated process, and consider the originals now omnibuses.
The sequels to Sailing to Redoubt were not planned, I just felt like writing more. The same goes for the sequels to The Darval-Mers Dossier they just, well happened. On the other hand, the Nine Star Nebula Mystery/Adventures stories where planned as a series. There was a "commercial" reason for this; they were something of a sequel to my best selling book by far, The Bright Black Sea. They were a return to that setting and featuring a younger version of a character from that book. But, as I said, none of my books are best sellers except relative to the average self-publishing author's sales. And still...
So why have I, and many other writers, written so many series books when it's such an iffy strategy? There are several reasons, chief amongst them, at least for me, is laziness. Subsequent books in a series are half the work, or less, of a new standalone story, since the characters and setting have been invented and developed already. All a writer needs to do for a new story is think of something else for them to do. However, it is also true that many authors have epic stories in mind, that they feel need three fat books to be told. I'm not one of them.
In addition, there is a sense of familiarity, even kinship, with the characters and the world a writer creates. There often is a relationship that develops between the author and the characters they bring to life. Characters develop agency, and with it, a sense of friendship and responsibility, a feeling that you own them more of a life then that one book. That said, many authors are cold blooded murderers of their creations, which I can't understand, but I guess it takes all kinds...
Both of these reasons, the laziness and the relationship between author and characters are independent of sales, and may well explain why a series is a series out of the gate, even prior it is released. Hope springing eternal. Writers hope that readers will buy the books, and love the characters and world as much as they do, and will demand more books. But that is almost always a bird in the bush at the start of a series. There are simply too many books being published for this dream to come true for many writers.
As I said, most of my books are part of one series or another. Of the 20 books (24 when counting The Bright Black Sea and The Lost Star's Sea as a six book series like it is in Amazon and Kobo) I've written, 8 books that can be considered standalone novels. But even then, there are relationships. Some Day Days and A Summer in Amber are set in our world, though perhaps 60 -70 years apart. Keiree, The Girl on the Kerb, and Beneath the Lanterns are all set in the Post-Solar-Age. The "Lorrian novels", Chateau Clare, Glencrow Summer, and the upcoming Some Small Dive, are set within a couple of years of each other and not only share the world, but a few minor characters as well. Plus, only A Night on Isvalar, Some Day Days and A Summer in Amber are books that do not fit into my larger "universe" of star systems settled by slower than light ships set out from Earth's solar system spanning civilization of the Solar-Age.
But in my defense, my advice about not writing sequels is aimed at writers who hope to make money writing, which is to say, 99.9% of writers. I don't write to make money, I write to entertain myself, and hopefully, any readers who happen upon my stories. So, you see, I don't have to follow my own advice, because I'm not playing the game, as the game is usually played. Plus my sequels do as well as the originals, there is very little fall off, so if I feel that if want to write them, I can, and they will find their modest audience.
Though I said that every book in a series sell fewer copies than the one before, there can be strange exceptions. For instance, the third book in my Nine Star Nebula Mystery/Adventure series, Shadows of an Iron Kingdom, is the bestselling book of that series, and continues to out sell the others, for some unknown reason. Perhaps its title makes it seem associated with a game of that name. Or Gothic novels sell... And then there is the case of Glencrow Summer, which now consistently outsells Chateau Clare, though in this case, they were written to be standalones, so that less surprising that one book outselling any other in such a loose "series". Though why it does, is a mystery to me. Perhaps, once again, it is it's title or perhaps subject, summer being attractive. But I guess that's the business. Random House was named after the business.
I'm happy with my approach, which consists of writing books with open endings that I can add on to, if I care to. Plus, I like writing a variety of novels in a specific "world" where I don't have to invent everything from scratch again. As I said a couple of weeks ago, inventing new worlds from scratch is a lot of work, work this lazy writer isn't inclined to do these days. Anything is possible, but I'm pretty comfortable in the worlds I've written already. In any event, you should not expect to see rayguns and spaceships in my future books.
