Godot Super Scaling : 1.1.2 Hotfix
After doing more testing, I realized the multi pass method was not totally working. While it did look more clear, it lost most of the benefit of the upscaling, meaning it looked better at native resolution or higher, but noticeably worse at sub-native resolutions. I spent the day debugging it, and was eventually able to solve the issue. However, the solution dropped performance significantly and I realized it won't work with this algorithm, at least not without major changes to the way it works. Though the result I got today did look pretty good, there was no point in exploring it further since the performance loss was too great, and that is the whole point of the add-on.
So I have reverted back to the single pass method. But I was able to bring in all the sampling tweaks and things I learned from the multi pass exploration, so now the single pass solution looks way better than before and the performance is still at the original speed. Though it does not look quite as good as the working multi pass method, the performance is very smooth and it still looks better than ever. Both the Ella and Decay demos have been updated with the new solution, so feel free to give them a try if you want to see how it looks.