Dear respected Ken Samuel Kyllingmark,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, I'm sorry and I'm afraid, in SharePoint online there is no list template available in modern experience which allows adding subtasks.
We feel your pain due to this limitation and we’re really feel sorry for it. However, we appreciate your understanding. I do understand the inconvenience and apologize for it.
However if you are using a team site, as a workaround you can use classic "Tasks" list template in SharePoint online to create a list where you can add subtasks for each task in list.
Go to Site contents> Select New>Select App or select Add an app from settings (gear) icon> On "My Apps" page, select classic experience link>It will open classic experience "Your Apps" page where you can search for & add "Tasks" list. After create/add a "Tasks" list, go to Site contents and access the added a "Tasks" list. There you can add subtask.