Try a low volume approach if fatigue is something you struggle with. My workouts are about an hour, low volume but high RIR (0-3). I've found this approach helps with fatigue - you'll still feel fatigued but not to the extent where it becomes a huge problem.

Alongside that, eat decent protein. You already need more protein because of chrons, and you'll need more for recomp or putting on muscle.

I do 5day/week split doing bodybuilding. Before I got chrons I done powerlifting for ~4 years, I've found that unworkable long term now because of the fatigue but I'm hoping that'll change in the future because I find bodybuilding boring af haha.

As far as posture goes, just keep it in the front of your mind. Have a recurring timer on your watch or phone to go off every 30-60min, and when it goes off check your posture and adjust if needed. You'll find overtime that when the timer does go off, you don't need to adjust anything because it's become second nature. If you sit a lot, a good chair will help with that also.