Just a quick follow up, I realised that the terraforming in the valley was super tricky due to enormous height variations of the valley floor from periodic dips/ depressions. I also noticed that oddly enough the valley further west from deepden (i.e higher up in the mountains) is lower than east of it and the goldroad there transforms into a monstrous beast which with its painfully contorted body scares every traveller by quietly wailing "kiiiiillll meeee".
Fixing that is a pretty large terraforming effort, which would be further compounded by my desire to preserve Deepden and Yew.
Hence, unless the mods in their eternal wisdom can concoct a different solution, I will limit my terraforming to start just slightly east of Deepden.
In preparation I've begun filling holes in the terrain and raising parts of Ferren to lie on a flat surface rather than in odd ditches. With your permission I'll gradually move south to continue prepping the terrain like this.