The books really shift tone between books 3 and 4 and 6 and 7. I felt the show did really well with the tone shift, however, I do understand how people might not enjoy the results as much as I do.

The first book/season is such a banger because it's pretty great horror sci-fi at that point.

Eventually the whole protomolecule thing settles down, and afterwards you have essentially politics and genocide in space, which can be good but almost feels like a different genre.

Right - the first season's gradual zooming out (expanse) from a very focused murder mystery into a solar system-spanning event was amazing. It did feel like that event then got sidelined, as you say, in favour of politics.

I found it pretty consistent! The improved CG in later seasons also clearly shows.

The expanse drop was entirely due to budget cuts from amazon, who were going to cancel it but didn't because Jeff Bezos liked the show. They did get a budget cut and had to make some decisions about how to execute the story with more limited resources. They still did a pretty good job imho, but would love to have seen what they could have done with full budget.

If this is true, why couldn't Bezos bankroll the show to maintain its quality?

Because it was worth a little but not a lot to him? I'll pay $20 for an audiobook, but I won't pay $100 for one.

One might be surprised by the proportion math of one's $20 as % of one's earnings or net worth versus his, and how far the same % could go.

Because billionaires can't love anything more than they love money.

Sounds like you're more obsessed with money than he is. He spent a billion dollars on a terrible Tolkein show.