I’ve noticed that a lot of other amateur fantasy don’t plan to write books, but to write a whole series.
This makes sense given the popularity of Game of Thrones, the Wheel of Time and others.
However, I think it’s a bad idea.
In practice, writers don’t decide whether there’s a sequel. Readers do. And having the original written with a sequel in mind doesn’t make a sequel more likely.
The Hobbit, for example, is obviously not written with The Lord of the Rings in mind. The Hobbit makes no mention of Gondor, Rohan, Mordor, or rangers. The elves and goblins/orcs are very different, and so is the ring: in fact one chapter had to be rewritten to remove a blatant contradiction between the two books.
Star Wars is another prominent example (despite George Lucas’ claims to the contrary). Darth Vader and Anakin Skywalker are obviously different characters in the first film (Ben Kenobi specifically says so). Equally obviously, Princess Leia and Luke Skywalker aren’t related.
I’ve sometimes heard people justify particular elements because “this has to be there for the next book”. If you have enough of these, there probably won’t be a next book- and if the book is good enough people will be OK with some inconsistencies.