Could he have gotten tired of the serialized story he was telling and run out of steam? (Which would be completely fair -- the world is great, the serialized story is the least interesting part of these books.)
See my spoiler edit to my previous post if you like.
I think he's written himself into a corner, and he's struggling to plot a path forwards, GRRM style. I don't think he's out of steam exactly - usually when that happens with an author you can feel it in how they're writing, but actually part of the issue with Amongst Our Weapons is that it has a ton of energy and forward momentum (imo) and then... it stops. Like he intentionally said "Oh I need to stop this book and put that stuff in a different one". But there isn't a different one.
Also he's falling further behind the real world, time-wise. Rivers of London is set in 2012, and was finished in 2011. Up to Foxglove Summer, all the books were published within a year of when they were set.
But then several books are set in 2014/2015, to the point of referencing specific real-world events from 2014/2015, and then both False Value and Amongst Our Weapons are both early 2016 (Jan and April IIRC). And False Value is very much a 2016 book - like what he's describing there is massive 2016 vibes (Amongst Our Weapons less so, it's more timeless).
And I think this timeline is screwing him as well because he's not the kind of guy to abandon it, but now all the characters ages have to match it, and he has Abigail and
two very smol demigod bebes for whom their exact age is going to matter a LOT (where to characters already in twenties/thirties/immortalities it might not) for a while.
The real solution is probably a time-skip - he could skip right over COVID and I don't think anyone would fault him for it. But even to skip to 2022 is 6 years, going from Abigail at 16 in Amongst Our Weapons to Abigail at 22, almost as old as Peter was at the start of the series (25) and
the babies would go from 0 to 6. I mean actually I write that down and it sounds like a great idea. But will he, like GRRM did, get trapped in a web of his own needless detail and precise events? I kind feel like he might.