IMO even with all those incentives I just don't see the typical age at first child getting to 16-24 in any Western country even with strong incentives.

Not just because of costs, but the harder to modify cultural reasons like people valuing independence, "casual dating"/FWB more common instead of dating with intent to marry, less social pressure/shaming from parents ("why don't you have a husband/wife/kids yet?!"), freedom to "find yourself/travel" or "get sexually experienced/have a ho phase before settling" for years after college, etc.

Most people don't want to go from college to marriage and kids right away these days, and a few years of this type of casual dating/exploration after college eats up a lot of prime reproductive years like you said. But these cultural factors are very hard to change especially with the absence of religion/strong communities these days.