Some good thoughts. I do agree that the 'trap' against DG is with assaulting too soon. Some of the DG units do shoot shockingly well though.
I have been playing against DG with my Thousand Sons, and the point of mentioning that here is even with -2AP bolters, it's so difficult to adequately damage them with shooting at T5 and -1 Damage. This is partially why I've swapped out my Redemptors for Macroplasma, always overcharging. That 3Damage is at least a bit annoying to them.
My second last game against DG was with my Ultramarines, and I probably made the mistake of taking Oath as a secondary against them. It's nearly impossible to hang out in the center of the table with them (with my UM anyway). You can't trade effectively with them without SERIOUS assault capabilities. I even had my relic Seal of Oath targeting the 10 Blightlords... no dice. I just could not damage them enough, and bolters are hot trash against them.
For my scars I'm thinking of going back to some Aggressors. Combined with the Chaplain on Bike, I hope to get +1 to wound with flamers/bolt gauntlets working. D3 fists +1 damage actually isn't that bad.
Also I tried a few times... it was a goofy thought, but it worked.... I put a Repulsor in Encirclement, for 1 CP which had Assault Intercessors in it (Not sure if this is legal), it had a great effect though.
Finally another new idea.... I've been hooked on Vanguard Vets with shield/claw, but I'm actually considering Dev's in Rhino's! Use the Rhino's as cover, get the Rhino's up the board, in range. Then use the strat to get the squad out and fire without penalty (I believe that's how it would work. Correct me if I'm wrong.)
The squad would be loaded with MM/Grav.