This issue goes way beyond choosing who will bat at 1, 2, 3, 4. Whoever we put at 3 or 4 or 5, they will struggle because of technical flaws, mindset issues, or simply lack of familiarity with the situation/position. Nasir Hossain is a different batsman batting at 6, 7 versus batting in the top order. I bet Mahmudullah will have the same problem. During Siddon's time at least Imrul Kayes was giving a start and was averaging close to 30. Thats because he was familiar batting at the top, had reasonable technique, and most importantly was batting with a specific plan or a goal which was to see out the new ball without playing too many rash shots. Imrul was also shielding Tamim from the good bowlers, rotating strike and playing perfect foil.
Number 1 we need a batting coach to work with our flaws. Secondly every player must know their role. Anamul shouldnt even think of slogging early on, and should do whatever to see out the new ball. Thirdly these guys must bat in these positions for their domestic teams. And if they fail at 3 or 4, we shouldnt just chop and change. Give them time to settle in. Like let Nasir play more against the new ball. Eventually he ll be fine