Main in combat sneaks at 50 are using mask of night and surveil.
Bloodlines gives you honed reflexes which is a short duration haste.
When it comes to a training skill I don't really think any one skill stands out honestly. Eolorns is kinda nice at 40 but it doesnt really give you anything over disappear at 50. Elaa's throat rip and Vladmin's Slaughtersault have some potential depending on how the combat changes go and all. The one I liked the most was Elaa's perforating slice which in the 30s was great because it was on a shorter refresh from assassins blade but post 40 condemning blade really became the better long term art to use. The same goes with the lower level versions they are often decent for their tier but they don't have a long term impact really.