I'm not really sure how slow you can effectively fish dipsies,but they are made to plane down and out at speed.And when they slow down or stall they sink like a weight.We usually do not start using dipsies till the spoon/harness bite starts when water temps hit 50-52 degress.By that time our trolling speeds are around 1.8-2.2 mph and the dipsies work just fine at that speed.
As for inline boards,you can use them at very slow speeds.Guys are using those boards right after ice out and trolling 1.0mph.You can even stall boards to get bites too.We have the tattle flags that pull down on all our offshore inline boards.Think that is best way to go.
Last season we did really well trolling bandits in late April thru May.Sweet lead was 70 and 80 unassisted.We were running 6 rods,so had 90,70, and 20/25 w 2oz on one side then 80,60 and 20/20 w 2oz on other side.The longest leads on outside boards and weight on inside boards.I switched up those inside boards between 20/20,20/25, and 20/30 with 2oz depending on what was catching.