Viewing originals on device with limited storage

I’ve been testing Mylio to decide whether to pay for the Premium Plan. I am trying to understand how to view photos in full resolution on a device with limited storage (my iPhone).
My expectation was that while browsing, the app would use the low-resolution previews, but once I tap on a photo, it would immediately download and cache the original from cloud storage (I have a vault on OneDrive). But it seems to only be showing me the low-resolution preview, which is not acceptable.
The only way I have found to see the original is to tap Menu>Download>Originals for that photo. But I don’t want the originals to always be stored on the device forever, I just want to be able to see my pictures right now.
Is manually downloading the originals and then deleting them really the only way to view pictures when storage is limited?

Keep in mind that Mylio has three different resolutions: Thumbnails, Previews, and Originals. The Sync on Demand feature will automatically retrieve a “Preview” version when you tap on an image (not “Original”):

But Preview resolution should be good enough for detailed viewing even on an iPad screen - and importantly you can EDIT a Preview image. The Sync on Demand feature works almost instantly for me, retrieving from vaults here on my local network. I’d expect it to work the same on a cloud vault, although potentially slower (but I haven’t actually tried that).

And yes if you still really want the full Original version, you have to manually choose Download Original.