I discovered the Infinite Timeline last year while looking for book series that mimic the MCU concept of shared universes. And I don't think I've found any that get as close as Robinson does. I gave them all between 3-4 stars, none of them are literary masterpieces, but they're all fun popcorn reads with wacky ideas. I doubt I would have read them all if not for the promise of the shared universe, and I think the crossover books do deliver on that promise. I think Singularity was a satisfying conclusion, bringing together all the characters across the other 12 books, and a 5 star read relative to the series. Almost every character feels like a different person to me so watching them interact in the final book is delightful, however I do think Robinsons character work across all his books is pretty surface level. All in all, I liked how it wrapped the shared universe up, although the defeat of the villain is kind of anti-climactic?
My other gripes are again the author brings up the Infinite Worlds theory, but fundamentally doesn't understand it. Infinite worlds does not mean every world exists!! And also the author writing himself into the book while I thought was kinda cringe bc really, self inserting yourself into your book...but it was actually handled in a kinda sweet way. My gripe there is that it made no sense like Will creates the universe but somehow the author self insert is writing it into being so like..I don't even know I'm confused just thinking about how it worked and honestly felt like the author was really hand wavey about it just to self insert into the book.
I don't really think I would recommend these books to someone unless they like shared universes or like B action movies (I very much doubt this gets a big screen adaptation because with how saturated the market is these would def flop as adaptations. And the plots are way closer to direct to dvd like movies). However, I will most likely read what Robinson writes in the future.