"Mysterious Dragon" seems straightforward enough. No need to get all philosophical about it.
There's more accurate translations and there's more meaningful translations. "Mysterious Dragon" is probably more accurate, but its just a bit odd in English, same with "Mighty Dragon" (or its even worse translation, "Powerful Dragon") to be honest.
It might just be better to stick to the pinyin, since that is the only real yet readable (in English) translation of their official names. I don't think any official PLA sources have specified the English name of these platforms anyway.
And in my personal opinion, "Weilong" and "Xuanlong" hit a lot harder in English than "Mighty Dragon" and "Mysterious Dragon" even if you don't have a clue what it means.