In addition to the two actually dropped in Japan, the US and Russia have been close to accidentally firing nuclear weapons since then, in each case because they thought they were getting ready to respond to what they thought was an aggressive act. It's just not a good idea to have nuclear weapons at all.
So, instead of "no first use", I would just have a global agreement never to have or use nuclear weapons for any reason, not even for defense.
Weapons by their nature will continue to escalate as long as technology improves, so if nuclear weapons continue to be allowed, time will surely bring a weapon even more destructive to counter them. Whether you believe they've brought peace or not, do you want someone to invent an even greater weapon and test it as the next "weapon of peace"? I don't.
So, instead of "no first use", I would just have a global agreement never to have or use nuclear weapons for any reason, not even for defense.
Nuclear pulse propulsion should be allowed for spacecraft, however. This was outlawed years ago by the https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treat... but would be much more efficient than other currently used methods and safe if used with clean bombs in space. See: https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
Weapons by their nature will continue to escalate as long as technology improves, so if nuclear weapons continue to be allowed, time will surely bring a weapon even more destructive to counter them. Whether you believe they've brought peace or not, do you want someone to invent an even greater weapon and test it as the next "weapon of peace"? I don't.