Nearly done editing my manuscript when I get an idea to turn it into a series.

When you pitch the book that you are finishing up, from what I understand, you should pitch it as a standalone, but you can mention that it has series potential. Then, after you find an agent, you can bounce ideas off them for the sequel and/or prequel.

If you're planning to start writing another one in the series while trying to find an agent, I'd start with the sequel, since that's the one that most interests you.

ETA: Congrats on nearly finishing your novel. 10 years is a long haul, and yet you still stuck with it. Nice job!

If the other books are not written yet, I'd recommend pitching it as a stand-alone. If you spark interest with agents, you'll have plenty of opportunities to have a conversation with them about the series potential and they'll advise you on the best approach.

I recently expanded my project/series to fit a 30 year timeline (that used to be 25). I now have 4 eras (instead of 3). Each era has multiple books. If I can get an agent and get my first book sold, I will do one book at a time till it is time to no longer make them (lack of interest, I finish the series, whatever). My first book takes place in era II. I will finish era II then go back to era I. I will finish eras three and four in succession, after era I. Reason, era I is critical to explain some big picture items introduced in era II and to make sense of the last eras.

I note my project, because I think scope is key. If you are doing a trilogy, it may disinterest people to read a "prequel" book before the closure. In my eras, each has every element of the Gustav Freytag dramnatic structure. Maybe, for large projects, it is okay to do a prequel (like the Star Wars prequel trilogy, eps 1 through 3). For 3 books, it may be a detriment to do this. I have no idea, but I wanted to share scope and how scope might be an issue if someone discusses any of this with you.