Sleeping position is most likely to not influence or interfere the PE process. Since we can’t say for sure, we have to use some logic over here.
Sleeping with your belly down could turn to be very uncomfortable because when the nocturnal wood kicks in it will press the penis against the mattress or your body.
With belly up you allow enough room for the involuntary woods but against the gravity. Since the penis is considered an “extremity” and the blood isn’t “prioritized” for the most of the time, I would be staying on favor of the gravity to facilitate the blood irrigation.
By logic and elimination we can assume that sleeping on our sides could be more beneficial: room free to blast the woods and kinda of in favor of the gravity.
About the PE and gym timing, I would save the PE for last. If you PE before, blood will rush towards the muscles and most likely will stay there for a while, leading you to some natural penile contraction. But again, don’t think this will affect the results, it will resume as personal tastes and preferences.