Hey! So this project is obviously of great interest to me since it's right there and the ideas of connecting our projects somehow. I agree that Sunglass is obviously an Andal house and Sweet probably a Firstmen, so since they're both under Stannis, so assumedly these are placed appropriately, then it makes complete sense that House Sunglass conquered Sweetport from House Sweet. Aeks and I chatted some on voicechat the other day about and how it might be interesting to style the Sweet Castle somewhat like Sweetkeep (as an homage akin to the Manderlys building New Castle like Dustonbury). Aeks also mentioned with that that York Castle and especially its awesome Clifford's Tower might be a great inspiration with your old-new castle idea.
I love the idea of a sept built on the remains of a godswood. But also agree that there should not really be too many septs and no septries since I plan on having a motherhouse near Sweetport to the North and to the South, there's already a large septry by Parchments. Plus the dearth of septs would be evocative of the area's firstmen roots.
With your terra and houses, I see you are taking some inspiration from the Canadian Atlantic as I am. An old Scots colonial style would be great to see and would provide a good transition from the Norman England Crownlands to the more "rustic" Eastern European style that seems to be taking form in the Stormlands. I also agree that your project definitely should be a bit more downtrodden with the idea that its not desirable land (thus why the Andals didn't conquer it fully). It would also show the stark contrast between these people and the affluence of Sweetport-Rambton (money off shipping and sheep) and Parchments (House Penrose is noble enough to twice marry Targaryens). They also don't need to really match the middle/upper class look of Sweetport since that is a very isolated, unique style.
Just one big part of your plan that I would like to suggest changing, and that is having your "Sweetwater/Sweetstream" either head North and out into Sweetport Sound or have it bifurcate at the lake/reservoir with one branch as is and one heading North out Sweetport Sound. Otherwise very excited, great job!