This is completely self imposed and a mistake in terms of how you approach Dynamo. Doubly so when you add in the next bullet point. It’s like saying ‘I need to build a complete for a large multi-family tower in Revit but without using any links and by the way I don’t know anything about things in the systems tab.’ The use of AI to try and get code is also going to slow you down (significantly compared to packages) and do more harm than good.
PolyCurve.ByGroupedCurves will build closed loops for you on this - in some cases you will have to extract the curves and manage them as a group goi g forward. You will have to manage list levels and lacing throughout the rest of the graph though. Do the primer exercises to learn that as AI tools will not help.
PolyCurve.OffsetMany has a boolean control for filleting outside corners or not.
I can not see where you used this as your graph image is too zoomed out. Read how to create an image of your workspace and highlight where you aren’t getting the result you are after. My gut tells me you should be using a Wall.ByCurveAndLevels node not a high based method though.