An approach I like to make my Drupal code more maintainable is to write less of it and move any generic PHP code into separate framework-agnostic libraries.
These could be in open-sourced packages installed from Packagist or kept within the same repository.
Having less Drupal-specific code keeps modules smaller and easier to maintain and upgrade.
If I need to upgrade a module from Drupal 7 to Drupal 10, I can reuse the generic code and focus on changing its integration points with Drupal to make it compatible.
