Mass immigration is becoming a zero-sum game. Japan's immigrants mostly come from China and Vietnam -- both of which countries have sub-replacement fertility themselves. Quite catastrophically so in China's case.

Now in mid-2023, the only countries with above replacement fertility are not "modern" societies in any sense of the word. e.g., even Bangladesh is now sub-replacement. Mass immigration from places with still-above-replacement fertility into countries like Japan and the UK is a recipe for disaster.

So #2 isn't happening, and #3 should be disfavored on ethical grounds. (Robbing Peter to pay Paul is at best ethically neutral; importing a natural underclass is to everybody's detriment and can only result in upheaval.)

#4 is the best option on the table.

Of course, there are additional ways by which the decline might be managed. Obviously pension and retirement schemes will need to be recalibrated, and older folks should be encouraged to move from the decaying periphery into large regional hubs where they can take advantage of support and medical services.