snubber circuit or opto isolator with relay
Adding a snubber over the optoisolator won't help as the relay coil is not an issue here. There already is a diode snubber for the relay coil.
Also, the optoisolator does really nothing, as both sides of the optoisolator shares the same supplies and ground. It does not isolate.
The snubber needs to be over the relay contacts, as they arc and spark when the load is switched.