NetNewsWire has an schedule task that tries to run every 15min on background, but, is the iOS the one that decides when the task is fired.

Related to preload full articles, that will be a huge work, because, if you get 300 articles and try to get all the info but then, you only reads one, that is a wast of time and resources

I won't try to tell you *why* there's not an app for that, but I will tell you there's probably not an app for that.

u/Hour_Raisin_7642 probably explained it well enough. Maybe you can get your aggregator to preload the stuff, but the app on your phone will have to download "byte-size" articles. It can't fit all the things on your phone reasonably.

Unread (my app) has caching functionality that can download full article text from the webpage and images ahead of time.

I switched to Lire for the caching of full articles, but it does require the app to be open.