My question to you - what is your solution?

I can't speak for anyone but myself, but my list of political objectives for the United States on this topic is to shut down immigration (which has popular support, and it should, since almost nobody in the US except the capital class actually benefits from it) and make a real effort to both secure the border and remove illegal aliens, which is almost as popular and thanks to the response to Corona we now know is very possible. I would also consider other measures like encouraging other groups to emigrate to their countries of origin, or revoking citizenship from people who got it illegitimately like as a result of birth tourism (which is pretty common but profoundly silly - imagine applying the mechanics of a tower defense game to the physical integrity of the nation) but when you look at birth rates and population trends the previous things are actually all that's necessary to make the US start getting Whiter again, although it would take many generations and there are limits to how far that will go. It took us almost 60 years to get to this point, and I am willing to wait as long to fix it if need be.

Do you want to outbirth all the other races? That would require to have like 5 kids per family and would decrease quality of living, while essentially the whites still barely breaking even.

I do support having a large family for religious and personal reasons but I'm not under any illusion that it would be enough on its own WRT demographics (though it certainly wouldn't hurt,) and I understand it's not desirable for everyone. I'm not interested politically or morally in forcing people to reproduce, or not reproduce, if that's the implicit question here.

Do you want Western countries to be 100% white?

Ideally it would be at or very close to 100%. Multiculturalism always results in conflict and resentment and minimizing it is better for everyone. Small minorities usually aren't that big of a deal though, and for the USA specifically Amerindian populations would at least have to be considered.

But then what's a guarantee it would remain that way 100,200 years from now? Immigration policies change.

So? It's still undesirable and I'd want to do our best to keep it to a minimum. The fact we might fail isn't a deterrent in this and I don't really get how people could think this is some kind of gotcha. You are right though that this adds an element of urgency, which is why I think we should be trying to get incentives to leave on the table rather than just settling for ending mass migration.

Wouldn't a better solution involve making sure the overall quality of life rises enough so that the demographics slow down all across the world, rather than just in the west?

This can happen, and in fact is already happening, but that doesn't change our situation.