Synced content via iCloud Drive
In addition to syncing your reading sessions, Panels can keep your content synced between devices. Panels has support for multiple libraries in addition to having the current device library.
The trick to having your content synced via iCloud Drive is to choose a root library folder stored anywhere in iCloud Drive.
To add an iCloud Drive library, head to the library tab, go back to "Content" and tap on the "..." icon, then choose "Add a new library". From the files app modal, navigate to any folder located inside your iCloud Drive folder. You can also create a new one if you don't have any comics in iCloud Drive yet. Choose the root folder and tap open. Panels will add that folder as a new library. From that point you will be able to navigate that folder within Panels, and use it as a regular library. Panels will not download any files to your device until you tap on them to read.
Panels needs to download each file once to extract the cover from it. Once extracted, the full comics will be deleted from you device and the cover will be placed in a folder and synced, so there is no need to download that file again until you read it.
You will need to add that library to your other devices if you plan to have that content synced between them.