I mean, there are really only two viable solutions if you want to minimize the development time spent on the problem - 1) add servers to the most densely packed DCs and drastically up the incentives for transferal, 2) add wards to packed servers without the ridiculous insistence on complete parity.

Being honest, I think adding wards to packed servers is legitimately the only realistic near-future solution. You're going to struggle to ever get a significant amount of people off of Balmung, for instance, no matter what you offer - not even if it becomes nearly unplayable. Only answer to a supply crisis where you can't stem demand is to add more supply.

Of course, in a perfect world data is free and instances add no overhead and are infinitely scalable so you could infinitely expand availability forever. It's also abundantly clear that we don't live in a perfect world and this game doesn't easily support that just yet, or it would have been done years ago. It's a huge systems engineering task, big enough that there are no practical limits on what the solution could be and big enough that you can't really pin all your hopes on it. It's almost certainly in the cards for the future, and I fully believe Island Sanctuary has been a grand data collection experiment towards it... but there's just not a magic "Add Instanced Housing" checkbox that they've been refusing to click.