I would say post-industrial economics, a radically declining infant mortality rate, and increased life spans have a lot more to do with the declining birth rate than a decline in religiosity. Having large numbers of children simply isn’t economically feasible or desirable in a modern economy - you can’t put them to work (it’s illegal and their labor isn’t worth much anyway), and it is very expensive to raise them for 20 years with a high standard of living and with the degree of education required by our economy.
Having large numbers of children in the West is almost always subsidized by the welfare state. That means it probably isn’t rational behavior for our society - not unless you’d rather go back to an economy where most of us do menial work in factories and on farms.
I don’t dispute economic incentives and pressures (driven by high taxes as well as the factors you identify) as the driving factors for declining family size in a modern economy. But I distinguish between the decline from ten children down to, I don’t know, three or four children, and a further decline (by choice not inability) down to one or zero children. At a certain point, I think materialism, self-centeredness, family breakdown, a general feeling of despair or purposeleness and similar factors have a significant impact when family sizes get down to the anemic range, and that such an outlook is tied to a general decline in rigorous European-based Christianity.