The Long Utopia: This book had a few things going on. The plot strand about Joshua Valienté's father and the 1800s past was a chore. The main plot of the book, however, relating to the silver beetle world, was great. A mystery! With a solution! And the ending was perfectly satisfying. I think this might be my favorite book of the series – if only due to the comparative lack of Joshua Valienté.
The Long Cosmos: This book just did not work for me at all. Mostly, it just felt like an exercise in wrapping up loose ends, with a giant red herring in the form of the Thinker. (They build a giant machine! ...that maybe possibly suggests to them to try to step North. Which has already been done by humans before in the previous book. Are you trying to tell me no-one tried it? Not even the Next with their emergency back-up super-stepper??) As I am still in my 20s, the theme of aging did not resonate. The Next managed to be even more turbo-obnoxious. The new characters introduced either became non-entities after one chapter (the GapSpace duo) or were annoying (Jan Roderick). I recognize the authors had to wrap things up, but the end note being "grandbabies ever after" instead of, say, passing on the torch (e.g. Indra Newton being part of an expedition to chart the Long worlds and search for alien life) was bleh. Also everything was just much too pat.
Overall, I think my biggest issue with the series is that it has a cool premise and worldbuilding, but the main characters are mostly terrible. I liked Sally Linsay okay enough, but she's basically the only engaging human in the entire series.
Lobsang, OTOH, was interesting. The Long Cosmos has a few places where it gestures at being interesting, and both of them involve Lobsang: the first, when he's in his simulated Tibetan village, LARPing a novice monk, with an actual Tibetan Buddhist abbot helping him in his path to enlightenment; the second, when it's revealed that Lobsang does not actually own himself despite trying, due to the guy in charge of Black Corporation interfering. I'd have liked more of either of those, and in general, an actual exploration of "Tibetan motorcycle repairman reincarnated as an AI" and possibly some actual Tibetan Buddhism. The humans are extremely specifically more-than-default Christian – Joshua is raised by nuns, who also feature heavily as characters throughout, and one of the other mains, Nelson, is a reverend/vicar – in proportions that made me feel all :/ about it. Having Lobsang's Buddhism be treated as more than scene setting/part of a LOLtastic character pitch would've helped immensely.
Also, it really bugs me that both of the authors are British, but all the books feel kind of parochial in a US flavor: everything seemingly happens in the geographical region of the US (save for some tiny excursions to Britain). Like, c'mon. You mentioned those Indigenous Australians going walkabout in the Long Earth at higher rates than other ethnicities; maybe visit one in Long Australia. Have something happen in Long Kyrgyzstan. Visit a world where the Sahara is not a desert. There's life outside the US, FFS.