Something to mention, use quality meats. Inexpensive lower quality meats have little chance of success.
I'm not a pro, just someone who has been fiddling with smokers for 30+ years. Also take into consideration that my thoughts below are based on simple cooking with dry rubs at most. I don't sauce, mop, brine or turn as a personal preference.
Dry meat in my experience is cooking too long at too high of a heat. For fatty cuts of pork and beef I find just above boiling, 215 - 225, works well for me . Leaner cuts of meet do not benefit from low and slow. You would be served well from a good remote thermometer probe placed at meat level to monitor your smoker temp.
I use a dual probe remote thermometer. One probe for the meat and the other for the smoker. On the my ThermoTemp 340 I put the smoker probe on the third rack near the sensor for the gas valve. The grocery store oven thermometers that sit on the rack can be inaccurate. Probes can be checked for accuracy in boiling water.
I'm not a fan of brisket so I don't do them very often but the method is similar to doing a pork butt.
I cook pork butts by time and temp. Most times they finish around 196 - 200 internal on the largest piece. My typical cook times are 18-22 hours depending on what texture I want. At 18 hours the muscle will retain some firmer definition. At 22 hours the meat will be softer. 16 hours would be my minimum.
Pork butts and brisket have a stall point where the temp does not move for several hours. This is where all the magic happens as the connective tissues are broken down. You can't rush this.
Ribs are a mixed bag. I find that ribs in my ThermoTemp 340 can get dryer than I like compared to my
WSM charcoal smoker. I believe it has to do with the extra airflow required for the gas burner and the smoke wood vs a charcoal smoker. I am currently experimenting with some things to overcome this. For now, doing ribs in the gas smoker, I get better results if I smoke for 3 to 3-1/2 hours then foil and finish in foil.
I think most people when they start smoking meat want to try a little bit of everything which can become overwhelming when trying to understand how to cook different types of meat. It comes down to understanding your smoker and at what temp it needs to be to hit your target time and temp which comes with experience. Every smoker I have used has its own sweet spot.
For long cooks pork butts are much more forgiving than brisket so I recommend honing your skills on those first. For medium cooks, whole chickens and pork roasts are a good place to gain experience.
A final thought. My kids grew up on my pulled pork which I spent 15+ years perfecting. This is their gold standard for pulled pork. When we would go to restaurants or events that served pulled pork one of us would usually order it and my kids could tell you to almost the hour how long it was cooked/smoked and what type of smoker it was cooked on just by the texture.
With the ThermoTemp being my first gas smoker, having come from years of charcoal and offset smokers, it may take several years to get the results I want.