I'll get in on planning an itinerary.

Day 1: Arrive in Yangon in the morning; sightsee; overnight bus to Bagan at 20:00
Day 2: Arrive in Bagan at 05:00; sightsee
Day 3: Bagan; sightsee
Day 4: Bus to Mandalay; arrive at night (there are multiple buses every day, ~6 hours one way)
Day 5: Fly out of Mandalay

Or the OP could fly into Mandalay on Day 1, making a roundtrip to Bagan.