Code P0300, how can I go by testing out the ignition coils and injectors without taking out my intake manifold?

Was the code there before you replaced the plugs? Where did you buy the plugs from? P0300 is a multiple cylinder misfire. Commonly that would be something like low fuel pressure or a vacuum leak. However, you mention it struggles to start and when you give it some throttle it smooths out, that sounds like a vacuum leak or intake leak. If you have a way to read live data and can see the fuel trims it would tell you very quickly if that is the case. The trims would be higher at idle and go lower as the throttle opens. The reason why I doubt it's the coils or injectors is that they generally don't fail that way, normally you get a bad coil or injector that causes a single cylinder misfire, if multiple coils fail it won't run, same with injectors.

Okay, on the plugs, I have seen many counterfeit parts from online sources. I wanted to possibly eliminate that. The fuel trims long term are trying to pull fuel while the short terms are adding. Try this. Start the engine, let it idle a bit. Note those trims. Now rev it up to 1000 and note those, now up to 2000 and note those. If at idle you see the trims going positive (like those readings) then as you step up the rpms they start to go down until you get to closer to zero trim (A perfect engine would show the fuel trims at numbers that zero out, so maybe long term at 2% but short term at -1% or similar). Also look at the MAF numbers, with a warm engine you should see about 3.5 grams per second at idle. Normally you figure one gram of flow per liter of engine size as a starting point. Then as the throttle opens it should go up at a nice even rate. However, if it hasn't been cleaned in a while this would be a good time to clean it and maybe the throttle body. Just to see if it is related. Another quick test to see if it's a vacuum leak, on a cold engine, start it and then spray carb cleaner near the intake gasket area, around the vacuum lines and check the air tube between the MAF and throttle body as well. If there are no leaks the engine will stay steady, if it suddenly changes when sprayed, look for a leak in that area.

Okay, so the tank is at say 1/4 tank it sometimes stalls, but then restarts. That might be low pressure from the pump or a weak pump. You might want to do a fuel pressure test to see if you have 51 PSI at idle. If it's a weak pump it would also act like a vacuum leak and can cause a P0300.

Injectors won't do that. Weak pump or a crack in the pressure line by the pump would do that.

You can get a pressure test kit at most parts stores as a loaner tool. On that one you test the pressure by disconnecting the line at the fuel rail and putting the tester inline. To remove the pressure before testing you remove the fuel pump fuse and then try to start the truck. Once it won't even fire, you connect the tester and put the fuse back in. Then run the test. https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator