@JClarke
I would have stuck to the M1 but there would not be enough cooling capacity for a 3900x without going 240mm AIO but I am not a fan of them due to the buzzy nature of the pump.
The D15 keeps the 3900x around 77c under AIDA64 stress test and is silent in light to moderate load. The GPU fans do not even turn on most of time as the bottom fans do a great job of keeping things under the the fan stop threshold.
The only thing that I would liked to see changed is if they substituted the top handle system for additional fan mount/ventilation. But that's personal preference.
I have attached a photo of it next to the M1.