How can I my sync webmail contacts with the K-9 mail app on my Android?
You can sync contacts from webmail to nextcloud and then to your phone:
https://howto.disroot.org/en/nextcloud/apps/webmail
https://howto.disroot.org/en/nextcloud/sync-with-your-cloud/android/calendars-contacts-and-tasks
Thanks a lot. I’ll try the instructions at those links and let you know how it goes.