The above applies to authors, bloggers, vloggers, and others who label something Part I or Part 1. For authors, if you are doing a multiple book series, it’s best to have all the parts ready to go before you publish your first volume. From a traditional publisher standpoint, if they accept your work for a trilogy or multi-set series, they tend to want you have at least the first three books ready to be published. That won’t publish them for a while, but from the various authors I see posting in social media groups, they regularly make the point traditional publishers expected them to have several books close to done, other than editing, before they would accept a multi-set series.
For self-published authors, bloggers, vloggers, etc., it’s best to follow the same rule. Maybe if you make it big, self-publishing or traditional publishing, then you can break this rule. However, it comes with a cost. I haven’t read a very popular series. The author appears in no rush to finish the next book. He has also been criticized for the long periods for new books in between some of his earlier works.
That brings up a second beef of mine. There is a reason that trilogies are popular. In a typical trilogy of reasonable sized book length, you will use every trope, character type, plot, etc. without overusing them.
I have a friend who would still be watching Star Trek: The Next Generation (ST:TNG) if it were still producing new episodes. It wouldn’t matter to him how many times they re-used every trope, character type, plot, etc. The show ran out of new ideas pretty early on, long before its last season. It didn’t help when they made a supposedly unstoppable group, the Borg.
That brings up another point – don’t add unstoppa.ble or over-powered species, individuals, or similar things to your work. If such an invincible force is added, you write yourself into a corner. Should the villain(s) be hard to defeat? Yes, but making them uber-powered detracts from your story.