Hello Karl-Johan Olofsson,
The information you provided regarding the Assistants API is accurate; it is a new feature that was just released this month.
If your considerations are focused on "connecting to my own data" and "how much data", then the On Your Data feature might be a more suitable option for you. The data for this feature is stored on Azure Blob and Azure AI Search, which essentially allows you to regard it as supporting an unlimited amount of data.
Best regards,
Charlie
If you find my response helpful, please consider accepting this answer and voting 'yes' to support the community. Thank you!