Upgrade from Drupal 8 to 9 is easy. Just install Upgrade Status module then you will see which modules or themes not ready for Drupal 9.
Install Upgrade Status module from https://www.drupal.org/project/upgrade_status, or direct install using composer:
#composer require 'drupal/upgrade_status:^3.8'
After activated this module then Reports - Upgrade Status. (WW)