Page Templates

Maybe you could use Pnp Site Core Library and something similar to site design scripts can be done for creating modern page templates.

Pnp Site Core Library has some customization options for modern pages including adding sections, webparts etc..

https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/modern-experience-customizations-customize-pages

There are some uservoice requests open for the same already

https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/18245221-page-templates-for-modern-pages-and-libraries-that

https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/20037172-page-layouts-in-modern-pages