Thanks for the pointers!
I think I’ll define two variables managed=0/1 and managed_suffix=“”/“-managed” in the way Michael described in the second approach so that when I want to use alternative files I can refer to them as e.g. src=resolv${managed_suffix}.conf and when I only want variations in a template I can use {%if managed %} there.
That should make it easy to differentiate the cases without resorting to only_if statements.
Regards,
Dennis