Good details! I would probably suspect that minor oxidation from transfers is what is diminishing your hop flavor/aroma. Getting and keeping bright hop flavor and aroma can be a somewhat tricky ordeal, depending on how serious you want to get. You might try eliminating the secondary and just dry hop in primary. Thats an easy thing to do that might reduce a little o2 exposure. Or when you transfer to your dry hopping secondary purge it first with co2, and use a secondary vessel that is shaped to limit head space, like a carboy when full to the neck has just that little circle of exposure but a bucket is wide at the top.
If that doesn't get you there you can explore things like closed transfer, kegging, spunding, etc.
Not saying it's not from oxidation, but this was my 13'th brew and I've had other IPAs come out with lots of the intended taste and aroma. On the same note, this was the first serious cold crash I've done so it was in the secondary carboy for about a week longer than usual, which could give it time to oxidize, although at ambient temps below freezing for that long I wouldn't expect much if at all oxidation happening in anything.
I do like using a secondary with any style of beer to reduce the particulate in the beer. I plan from now on to cold crash and not use gelatin, unless I'm making a non-hoppy beer that should be clear.
Purging with CO2 is a good idea, but what's the most cost effective way since I don't yet have kegging equpment? I've read about doing that in a keg before kegging.