It will be an amazing trip but best if you can move it earlier, Musken.
March is the start of the low season especially for Luang Prabang when the rains come & heat/ humidity start becoming unbearable. But with climate change, this maybe delayed?
We were at Amantaka in Feb this year & had an absolutely wonderful time. Weather was still relatively comfortable at night though got scorching hot in the afternoon.
As for Siem Reap, Amansara , you will appreciate it as a base for the many possible explorations. Was there in Jan, many moons ago & even Angkor Wat in the morning got a bit draining with the humidity.

Perhaps heat & humidity don't bother you, if so , March will be fine otherwise better shoot for Jan or Feb latest.
You will be glad, as usual , to be at Amans