a quality gun belt is essential. Equally as important as the holster itself. 5.11 Tactical makes quality & durable (though not necessarily "pretty") gun belts that retail for about $35. You can often find these at half that price on eBay. EOTAC, Wilderness Tactical, and Blackhawk also make quality web belts that are strong enough to carry a gun all day.
As for holster, Cross Breed Supertuck, CompTac Minotaur, Durham holsters, Old Faithful holsters, etc. all offer similar products, all will work to keep the gun tight against your body and reduce printing.
One trick you might try is "stretching" your T-shirts a bit wider where they fall around your waist. Sometimes if the shirt is cut a bit too narrow or "clings" a bit too much, that can be a bigger problem than merely the "size" of the shirt. I used to just walk into a store, grab a shirt in my size, and walk out. I now have to try clothing on to make sure it will fit "just right" for concealment.
We've all been where you are. I'm 6-1 and about 275 with a well-formed middle-age gut and I'm able to conceal full-size Glocks, M&P etc. under T-shirts when I need to. But I do wear a shirt that is one size bigger than "normal" and don't worry what it makes me look like. When carrying a gun closer to size of your XDSC (my Glock 27, for instance) I can conceal it in Supertuck under a T-shirt that is my "normal" size.