I would like to get a reliable high quality mid sized 9MM auto with a thumb safety. I have a Smith 3913 but would like something with a little more capacity and a better trigger.
I would appreciate any suggestions.

LJS, if money is not a major factor in your purchase, a Sig Sauer P226 Legion in the Single Action Only (SAO) version might be an option. It is a bit larger than mid size but an excellent pistol. In addition to this pistol, I believe the P320 series can offer a safety as well. At least it is on their model that has been selected as the new handgun for USA armed forces. Good luck.

I am a big fan of Sigs but those you mention are both too large for my application. I want something I can carry in my front pocket in a pocket holster. I have a Sig 239 that would be perfect if it included a safety.

The Sig p365 is now offered with a manual thumb safety as an option. I have a S&W Shield and Sig p320 Subcompact both without safeties but I always use a holster that covers the trigger guard whether it is carried OWB, IWB or in my pocket. The p320 subcompact is pretty much the largest that will fit into a cargo pocket with holster. I like the Remora holsters for this carry.

Since you are already familiar with the Smith platform + only want more rounds there are a lot of model 59's out there for a few hundred bucks.