Engine cranks but has trouble starting

That is a classic sign of a failing fuel pump check valve failure. Unfortunately the only way to replace it is to replace the entire pump. There are kits that install a new valve in the fuel line itself but they can leak and require cutting the fuel line to install. Plus if that valve is sticking or broke it's likely the pump is also getting weak.

So just replace the entire fuel pump?

Yes, the check valve is part of it. It retains fuel pressure in the line to the engine after you shut it off. If it leaks down the pump has to fill the fuel lines after it has sat a while. There is an easy way to confirm it's the check valve. After it has sat long enough and it will take a few seconds to start try this. Turn the key on, wait 3 seconds, turn it back off. Don't try to start the engine. Repeat that about 3 times. That will cycle the fuel pump on and off to fill the lines. After the 4th key cycle, try to start the engine, if it starts up normally and runs fine, it is the check valve. I've had a couple GM's where I drove them that way for quite a while before replacing the pump.
Another test to verify it isn't a bad injector would be to try starting the engine with the gas pedal held to the floor, that would enter clear flood mode, it turns the injectors off but fires the plugs. This one would be more likely if you said it started hard but then was showing a lot of black smoke once it did start.