If you use Nextcloud, it could be a way to add these calendars in Nextcloud. Then they can be synched via DAVx⁵. This would mean one app being active in the background less, too.
And now being at home I just installed ICSx⁵ and tried it in combination with Simple Calendar. It works for me. ICSx⁵ needs the permission to access the calendar, then I can choose it in Simple Calendar via Settings -> CALDAV -> Manage synced calendars. Would still prefer the single-app approach, if possible.