Is my existing electrical feed to my detached garage safe?
First there are multiple code violations even if done in the 60’s when Romex or nmb did not have a ground.
Having 2 hot and a neutral was ok I believe until the 1999 code change. But the max breaker is 15 because of the 14 gauge wire.
If the 14 gauge was removed a 20 amp breaker could be used but that was the max when built and still is today without a sub panel.
The switches and outlets for 120v are only listed for 20 amp circuits a 30 amp is a violation (even if a 15 amp receptacle or switch they can be used on a 20 amp circuit and be code compliant).
Could this be made safe? Yes, change the breaker to a 15 amp OR remove the 14 gauge wire and go to 20 amp.
Install GFCI protection on both lines you have 2 120v 15 OR 20 amp circuits at that point and this would be safe and even code compliant.
What would it take to make it better? Meet today’s code?
I would suggest a sub panel but then you would need A ground would have to be pulled from the service or on a point from that same panel to the garage.
A ground electrode would need to be driven or other method of grounding electrode to the panel. Then you could leave the 30 amp breaker in the main, install a 20 amp breaker for the #12 and a 15 amp breaker for the #15. And that would pass (what you have should have been flagged on the home inspection for the reasons I mentioned).