Sorry to hear about these problems and your frustrations. I'm in a similar position with mine right now - it ran great until last summer when it started popping out the exhaust on driver side. Changed plugs, wires, O2 sensors and got a new tune and it ran great for a couple drives then started doing it again. Compression test went great but I couldn't get a good leakdown test done on it. It has been swallowing coolant since I built it and I only have 1,500 miles on it. Right now the heads are at the machine shop getting rebuilt and I am putting new LS9 head gaskets, LS7 lifters, lifter trays, pushrods, and rockers on it. Basically a top end rebuild. On top of that, I was worried that my hacked up engine harness may have been causing issues so I'm replacing that as well. I'll be into this project for about $1500 which is more than a 5.3 from the junkyard, but I didn't want to give up on this engine as it's a 6.0 LQ9 and I've done a lot of work to it already. I forgot to take a picture of the sad situation - parts laying around the truck, a half-taken apart engine, a huge coolant mess on the floor.
If money was no object, I would be putting an LT4 in it. The more realistic option if this doesn't go well is to pull the motor and rebuild the rotating assembly or even put a Gen 5 LT powertrain in it.
I understand your frustration. We put a lot of time and money into our trucks and to have issues like this is disappointing after so few miles. I chalk my experience up to the gamble of a salvage engine. I don't know how well the previous owner(s) maintained it, drove it, or how long it sat after being wrecked. The only way you can be sure that an engine is tip-top is to rebuild it yourself or buy a new one or one that has not sat for long and has low miles. Like others have said, stick with it, don't give up on it. It's just going to take time, money, and patience to fix it.
Worst case, you pull the motor, have the machine shop clean it really well (in addition to you cleaning it after you get it back from the machine shop), and you can rebuild it yourself. If the engine is as new as it sounds, you should be able to reuse your crank, rods, pistons, maybe even bearings. I would probably put new rings in it, oil pan gaskets, oil plugs/barbell, obviously head gaskets, ARP head bolts, and it should be in good shape. My dad and I just did our first start on a 5.3 LM7 we rebuilt it and it's a good feeling. This was something I wanted to do and the costs do add up when you're doing a complete rebuild and prepping for boost, but we can be sure that it was done right (or at least that a couple first-time engine builders followed all the right procedures and had the right tolerances). You can rebuild an engine in your garage with some pretty basic tools and a few specialty tools.