Wilmot’s End will generally be the worst way to obtain anything. Using War of Assassins, and then cashing in your Dramatic Tension at Wilmot’s End will be slightly better, but it’s still going to cost you 34 actions per Strong-Backed Labour.
Using the Bazaar Sidestreets (after building up supplies via Iron Box, Unfinished Business and conversion) will yield a Strong-Backed Labour per 17.4 actions (proportionately).
Using the Docks/Orient conflict card is more efficient if you need large quantities of Strong-Backed Labour, but it might require some investment.
Getting a connection up to 30 requires 465 CPs, and Connected: The Docks isn’t one of the easy ones. You can boost it by +5 CPs/action at the Carnival until 10, then use the connection item for +25 CPs/action (at a cost of 0.5E of glim) until 20, and then you’re largely looking for opportunity cards (which, depending on the card, will yield +5 to +15 CPs/action) until 30.
Connected: The Orient is more straightforward. You can raise that by meeting with the Gracious Widow in Spite at a rate of +10 CPs/action (with a bonus of 36 x Jade Fragment each time).
So, 47 actions to get Connected: The Orient to level 30 plus an action to play the conflict card means that if you need 4 x Strong-Backed Labour (and we assume you already have your Docks connection), each Labour is costing you 12 actions.
That’s worthwhile even if you only need three, but at two or less (or if you don’t have Dock connections and you don’t need enough Labour to make the investment worth it), you’re better off heading to the Sidestreets.