I've written three books in a series and held off querying the first book, and I'm glad I did. After I wrote the second and third books, I've come back to the first and still had the flexibility to change what I needed to fit the series.I have this debate daily in my head because my WIP is the first in a series. I plan on querying it next year (if/when it's done) but at the same time I'm still not sure I should query it. Maybe I should put it to one side when it's done and write a single standalone to try and nab an agent. Partly because I worry about getting a deal for a series but also because I've pantsed what I've got so far. Publishing book one before any of the other three are done scares me rigid because then I can't change something in it to suit a new plot idea in one of the future books.
Although my example is somewhat different because each of the three books can both begin and end the series, or be standalones. Each one has a different protagonist and there's a ten year gap between each book.