I am very impressed with your DIY S2D solution, especially the Network interconnect between nodes.
However, I am a bit confused when you stated that you used Samsung 850 Pro 256GB SSD for Caching. S2D requires non-volatile buffer cache SSD, aka. Power Loss Protection (PLP, a premium feature only have on Enterprise grade SSD, like Samsung SM863, or Intel S-series and P-series. Without PLP, S2D will not work as fast as it should be. Here’s the Technet article: https://blogs.technet.microsoft.com/filecab/2016/11/18/dont-do-it-consumer-ssd/
Have you test and verify the performance of your setup? Can you share the IOmeter result? As I cannot find PLP inside 850 Pro datasheet, I assumed that it lacks the feature.
Please kindly correct me if I am wrong.