Moving 240v disconnect box
The cheapest couplers are other things
Generally, couplers are expensive for what they are, and your desire for fixed, attached couplers won't be well supported (not least because wire bending radius rules makes it awkward/complicated).
So the most expedient way to connect four #6 wires to four other #6 wires, with isolation and fixed down in a box, will be to use a disconnect switch.    Which you say you already have, so &quot;that was easy&quot; :)
You glossed over this part pretty fast, so I'm not sure if you are handling this properly, but a 15A receptacle is only allowed downline of a 15A or 20A breaker, you can't tap off a 50A line and bring it out to a common receptacle.  You fix that by using a subpanel at that location.