Keeping your workout intense as you put it, requires a lot of preplanning matched with diet and a sleep regiment.
So, first off, training with weights themselves comes in all shapes and forms. As such, dead weights like plates, dumbbells, kettlebells, weighted apparel, weighted bars, and then support equipment like weight benches, a pulley resistance system, and other apparatus are all out there for your selection with purpose.
The purpose of this equipment is to develop certain parts of the body for a reason. In that regard, pitchers are a unique challenge and require a careful dose of intertwining of what - when - how much and a cycle to follow religiously using one kind of weight or resistance, then a stretching routine, then rest and hydration, then start all over again.
I’m hesitant to recommend a “heavy” weight training program for pitchers only because of the requirements on the body after the fact, sort of speak. Agility, quickness, a fluid motion combined with endurance and tolerance has been my coaching in the “weight room.” I use the phrase “weight room” only because it’s kind of one-size fits all description that’s understood to include dead weights and other apparatus.
My suggestion would be to work with certain weights tailored for the upper body platforms like arms and shoulders, then other weights and resistance for the midsection and core, and yet other weights and resistance for the pelvic, buttocks, legs and ankles. So, light weights for the arms and shoulders, say 2 - 5 pounds max, just do more reps of your preferred workouts. Resistance workouts demanding no more than 25 pounds for core/mid section. And then for the lower body select weights, resistance and support apparatus suited for you strength and ability to manage AND CONTROL without straining.
Again, I prefer a pitcher to go easy on the amount of weight - just do and increase the reps. I look for endurance and tolerance, controlled strength and overall health. Also, don’t forget to work in a meal plan with enough time for digestion and benefit, prior to and after your workouts.
By the way, it’s a good idea to tell someone when and where your doing all this weight training, especially if you’re going to be working alone. Getting hurt or sustaining a serious strain with no one knowing where you are is very poor planning.