You are right. If you try to use a Repeating Field in your record, it will be too messy.
The accepted practice is to create a new table for this particular data and add a 'Portal' of this new table to your layout. That way, you can add one or more time points to each record using the Portal. Each record in this new table can hold the timestamp, the data point, and a foreign key as well as a primary key of its own.
Do you have a unique primary key on your table record? If so, you can use this as a foreign key on this new table to link these two tables together using the Relationship Graph (see the menu: File>Manage>Database…).
For details on how to create a portal, please refer to the topic "Creating portals to hold related records" in the FMP help menu.
Thank you for the tip. I will try it and let you know what the result is.