You make it sound like only one issue will fix it. Other countries that tried to increase birth rate, needless to say mostly for nationalistic/racist reasons have so far failed. Look e.g. at Hungary. They try everything but the birth rate is still declining.
There needs to be much more affordable housing and public transportation, but immigration is also inevitable in helping mitigate the effects of the aging population. And I think migration is a good, normal thing to happen. Humans have been moving around all the time, otherwise we wouldn’t be spread all over the globe.
But there also needs to be an environment where people feel comfortable getting children when thinking about climate change and the future world that they’d bring the children into.
So there are many things that go hand in hand and what I read from your comment is more like migration problem must stop, build housing solution done.