Be mindful of the distinction between ‘births per year’ and ‘births per woman:’
Ceteris paribus, births per year could decrease due to demographics alone.
(I don’t like the label ‘fertility rate’ because it shifts the blame on biology, like eggs or sperm count, which arguably did not change.)
However, reportedly both are slumping in Japan.

To support your point, compare: Poland’s “economy” goes up, they’ll even be admitted to the G20 soon. It’s still a relatively “religious” country—yet the TFR is trending down.
— Changing Fertility Patterns in Poland: Urban, Suburban, and Rural Dimensions (Nov 2025)
The tale it were contraceptions and whatnot is humbug for an agenda. We do have them in the West for decades now, yet the birthrate is continuously slumping even across income brackets, i. e. no mere jump to a plateau. They have been available for as long in Japan, and abortions carry less stigma. Religiosity is and was also different in Japan.
You’re not making that point, but I don’t believe people who spend most of their waking hours on activities around work or work preparation (commute into a big city because you’re priced out of an apartment; recovery from workplace circumstances and uncertainty) are inclined to have children. Once you toil, in SK, US, and Japan, you have to have a high savings rate to have hope to escape that lifestyle and forced infantilization—which will move age-at-first-birth up, yet desire to get children (naturally) decreases with age, for men and women. (It’s at 3 for early 20s, plateaus at early 40s.)