Considering that Spoondles is selling 550Gh/s for $15000, D5 is cheap relatively.
They're going for around $11k now, and they're more than twice as efficient. Given expensive power, the SP is a better choice. If your power is cheap or free, D5 is a better choice. Regardless of either, I don't consider mining X11 to be a good idea AT ALL. DASH is indeed pumped by Bitmain and has little real backing.
You are wrong. Bitmain did not even ship their G2 miner (GPU based) until December 2017, and it only did 220MH/s at 5.5 J/MH. The E3 shipped in July 2018, and it only does 190 MH/s at 4 J/MH, a tiny improvement. Considering the DRAM requirements and shortage of available parts during the late 2017 - early 2018 timeframe, I don't suspect that other manufactuers (e.g. Innosilicon) had working parts during this time either.
Anyway, if there were any groups doing secret mining, they haven't released their miners. Inno and Bitmain are the only groups that have released their parts publicly, and only Innosilicon's are a marked improvement over GPUs (1.8 J/MH, 485 MH/s).