Hi all, yes, Apple makes it difficult for users and developers. The missing file system is really an issue.
But there are apps like File Explorer Pro (Skyjos Co) who can add Time Capsule as a storage place.
When I want to save an Aphoto file, I save a copy, click on the button in upper left corner, I think it is called data provider (sorry, I am German). Then I select FE Pro and inside FE I am selecting Time Capsule and the right folder.
To open a stored file You can do that inside Affinity Photo on iPad following the same way via FE or you can go to FE and click on the file and select open with ...
All within your WLAN system and not in the cloud.
On my Mac I am using Carbon Copy Cloner and sync my Time Capsule with my Mac every view hours. No I am not using TC for Time Machine backups, there are other external Drive for this.
The TC is always accessible in the network which is important for this method. Could be a NAS is possible with this way as well.
Let me be clear, the Affinity Photo for iPad is not designed to handle many files in one go, I do not consider this nice little App as a rival for Lightroom and Photoshop.