Hmm, I can understand your worry and confusion, but I would say, begin from the start.
If you have planned the books to be 1-2-3-4-5 then I would, personally, write book 1 first, followed by book 2, and so on.
I understand that you might have hit a writer's block on book 1, but just hiding it won't help just replacing it with something else. You will have to write book 1 at some point, right? So why not focus on trying to get it as you want. Read it, rewrite it, and when you feel that you need something different, reward yourself with writing a bit on one of the other series, before you return to writing book 1.
I personally have a book "Universe" with 6 series, the Stargazer Universe, where each novel has a different genre.
Five of these six books are ongoing at the same time, while the sixth follows up after the first five are complete.
How do yours merge together? How are they connected? I am not sure what your thoughts are, but my universe is connected like this:
Once upon a time there was a race called the Stargazers. They were the leading race within the universe, but as time went by they became fewer and fewer and was about to be extinct. They turned to the Primeval Chaos and asked to be saved. What the Primeval Chaos did was to take the surviving 3000 people and change them into a "system" called the Primeval Chaos System, which then got spread over the entire universe.
Their aim is to find a host (female) and help this host to reach the apex of the universe. When the system reaches level 100, they will be able to impart the heritage / legacy of the Stargazer's race into the Host's child.
the children of the hosts will inherit the stargazer's legacy and thus they will never die out.
The sixth book in the series is about a stargazer who survive, and how she, alongside the 5 children of the MC's in the previous 5 books, recreate the Stargazer's Race
My series are completely different genres, but they are in the same universe, and they all are needed for the final book to work out as I had planned.
1st book - Primeval Chaos System (Classic Eastern Fantasy)
2nd book - Birth of the Stargazer (Modern Cultivation)
3rd book - Havoc (Zombie Apocalypse)
4th book - Ascendant (Sci-Fi Fantasy)
5th book - Rampant Truth (Dystopian virtual reality)
6th book - Everyone meets up and the universe turns to chaos.
As such, my suggestion to you would be to sit down and take your time to think your story through. Consider how they are connected. Start from the beginning, and slowly move forward.
Making a book universe with many series in the same universe is often very challenging, but it is also very exciting. I have done it before, and are doing it again, and I really like the feeling of characters from one series appearing in another. I also think that it makes worldbuilding much more interesting, because you can expand it over many series, and show different living standards in the same universe.
Anyway, this is just my thoughts. I hope it can help you!
Good luck with your project!