I’m using ICSx⁵ 1.9-standard and DAVx⁵ 3.3.8-ose (thanks), both from F-Droid. In DAVx⁵ I am subscribed to 1 caldav (Personal) and no webcal. In ICSx⁵ I am subscribed to 3 webcal - these are all public holidays and only change about once a year.
So in ICSx⁵ I’ve set the sync interval to manual, this in confirmed by a message shown on the bottom “Automatic sync disabled”. The problem is whenever a network connection is available ICSx⁵ still tries to sync even when it’s not supposed to.