There is a massive amount of land available to build houses in Seoul

Relocation of the National Assembly, Blue House, and Sejong government complex
Military Academy site
Yongsan Park
Relocation of judicial buildings to Daejeon
Relocation of the Central Investigation Agency, Prosecution Service, and Police Agency to Daejeon
Converting existing government buildings into rental spaces after renovation
By releasing the remaining land and easing Greenbelt restrictions
And adopting a Singapore-style model where the state owns the land and only builds apartments for lease/sale, it would be manageable even for the 2030 generation.