I believe if you read the OP you'll see the answers..
Good point. I missed his 9mm statement and focused on my lack of knowledge of pistols meeting his other criteria. I like to get as much info as I can when someone is looking for help. What he gave us is not really enough, IMO. Without more info, we get 20 posts recommending things he may have no interest in. With accurate and more complete info from him, a couple of posts with a couple of suggestions each could get him headed where he really wants to go.
The single stack is his biggest limiting criteria in a DA/SA. Next is lightweight.
However, he did not mention how he intends to carry, and that can make a big difference in his choice. Weight may be less of an issue if he is not pocket carrying. If size is negotiable, that opens up other options.
The discontinued Sig P239 suggested is over 30oz loaded and is not small. Probably not good for pocket carry at almost twice the weight of his 642. The S&W 3913 is similar in size and weight. Both were dropped because they didn’t sell well. DA/SA steel framed guns fell out of popularity in the late 90’s as polymer striker fired pistols replaced them. That’s why it’s hard to meet the criteria he gave us. On what points is he willing to look at other options?
It might be good to know why he doesn’t shoot the 642 well. Perhaps physical limitations? How does that influence recommendations for other guns?
It is also helpful to find out why DA/SA is what he wants, since a 642 is DAO. How one carries often influences the decision to have DA as the first pull—safety while putting the gun away being one.
For instance, the Glock 43 with the addition of a “Gadget” for safe holstering might be an option.
Striker Control Device
– Tau Development Group
A Shield with a manual safety could be an alternative.
But really, suggesting all these alternatives not knowing why he thinks he wants what he described—that I’m not sure exists—wastes everyone’s time. Your criticisms without offering anything helpful are more snarky than helpful.