Hi @jops!
As far as I know, unfortunately, Klaviyo does not have a mass update API for custom properties. As you mentioned, you would need to iterate through each profile and make an individual update call for each one. This can be a time-consuming process, especially if you have a large number of profiles.
One possible optimization you could make is to batch your updates. Instead of updating one profile at a time, you could update multiple profiles in a single API call. Most REST APIs have a limit to the number of records you can update in a single call, so you would need to check Klaviyo's documentation to see if there is a limit and how you can optimize your updates accordingly.
Another option you may want to consider is using Klaviyo's Import API to import all the profiles and their custom properties in bulk. This would be a more efficient way of updating your profiles, especially if you are making large-scale changes to many profiles at once. You can find more information about the Import API in Klaviyo's documentation.
Perhaps someone else will answer whether it is possible, that's all I can help so far.