It’s already output in iso format, which is yyyy-mm-dd. If you want it output in a different format than that, you can use .strftime
{% set t = state_attr('switch.schedule_0ce9ef', 'next_trigger') | as_datetime %}
{% set h, m = states('sensor.zone_gesamt_time_total') | regex_findall('^(?:(?P<h>[0-9]{1,2})h ){0,1}(?:(?P<m>[0-9]{1,2})min){0,1}') | first | default(['','']) | map('float', 0) %}
{{ (t + timedelta(hours=h, minutes=m)).strftime('%d/%m/%Y %H:%M:%S') }}
Just keep in mind that all these format changes will make these entities harder to use in automations. If you simply stick to normal formats, with typical sensors in HA, the frontend will translate all times into your selected format.
@parautenbach is on the correct path saying that you should rework all your template entities.