I need a decent molded-leather belt holster with thumb break

High Noon "Sky High". I have the non-thumb break version, the "Slide Guard". It is a very nice holster.
http://www.highnoonholsters.com/Product_Line/_Sky_High/_sky_high.html

Andrews Custom Leather "Saddle Style"
http://www.andrewsleather.com/traditional.htm

For open carry, the thumb break is probably a good idea, but for the concealed carry user it is probably not needed. Do a search for "how to break in a new holster". You'll see guys commenting on how they've given themselves a "weggie" trying to get the pistol out of the holster. The retention of a well made holster is such that the pistol is not coming out unless you want it to come out.

I see you're in NC. Wild Bill's Concealment is an NC holster maker if you want to keep the money in state. He doesn't make thumb break holsters though. He does have a FAQ about the option though.
http://www.wildbillsconcealment.com/wildbill/shop/

Have you checked Galco? They have some good things.

FIST, again

FIST Holsters will fix you up. Multiple styles and options including thumb snap.

http://www.fist-inc.com

As the previous poster mentioned, the High Noon Sky High is nice, but be prepared for about a 3 month wait (was about a year ago).
I like thumb breaks, but it is snug in the Sky High-I don't know how much the Slide Guard might loosen up with age, but it will probably be plenty snug.

I also have a DeSantis Thumb Break Scabbard-some come with 2 loops, some with 3. The 3 loop models give some flexibility AFA positions, but are a bit more bulky due to the 3rd belt loop. You'll have to check your particular model to see if it comes with 2 or 3 loops. For your model, the Thumb Break Mini Slide looks like a nice option?

I also have a Galco FX Suede, but I see the closest thing available for the M&P compact would be their Cop Slide Holster?

FWIW, I tried a double mag holder from all three companies and liked the DeSantis much better than the other two. It has more of a shrink wrap design to it, and due to construction both mags have equal tension. Some of the others overlap the leather when forming the pouches, and you can't get equal tension on both pockets!

The DeSantis and Galco are easier/cheaper (eBay)/faster to find than something like the High Noon, but the High Noon is nice!

The "three slot" comment reminded me of the Simply Rugged "Tribute".

http://www.simplyrugged.com/the-tribute/

I've been making custom holsters for 15 years and can make what you're looking for.

S&W M&P9c OWB1 with thumb break

+1 for the simply rugged. I have several they are well made and very reasonably priced.