I'm sorry, I completely forgot that I posted this. Deleting the actual hosted feature service itself will delete the data.
But I don't want to delete an entire feature service just to delete the data, because then I'll just have to re-create it. Deleting records through the hosted feature service through ArcGIS Pro only deletes it from our view in Pro/Portal, but it doesn't actually delete any content because the space available never increases (this is after deleting thousands of records with thousands of picture attachments).
I did ask this question at the UC and the solution was to disable and re-enable sync in the hosted feature service settings in Portal. While that did work.....it also wrecked the feature service (from a Survey123 form). That solution might work on basic feature services, but I have some more complicated ones with related tables/attachments.
I'm not having an issue with backups as they are getting stored on our file share and not on the VM itself.