Best choice between flyback diode or an RC snubber
You have to decide whether it is current thru the contacts that cause the problem or open circuit voltage spikes that cause problems. What do the symptoms tell you?

What does a welded contact mean - think about this - does it mean it has just gone open-circuit and an arc has caused it to splutter some contact material up into some form of plasma that has welded the contacts shut OR, does it mean that the contacts have welded upon making initial contact due to long-term surface erosion of the contact giving higher-than-normal contact resistance.

I'm using your words here and extracting symptoms that make sense to the words you have chosen to put in your question.