As much as I like Poison, at this point it seems like Fighting is a better choice. The removal of the rock weakness is important, though not because of SR. Rather, the problem is that a lot of the Pokemon we'd want to switch in on carry Rock-type attacks. Flying/Fighting CaP1 poses a lot more threat to dangerous sweepers like Excadrill and Terrakion than Poison/Flying, and Fighting/Flying doesn't have to worry as much about being caught off-guard by a predicted Stone Edge.
Flying/Poison is not without merits, though. Immunity to Toxic is nice, and we have a much easier time switching in on Grass-types like Breloom, Virizion, and Venusaur. It also has the nice ability to retain its Fighting resistance even while Roosting. Taking only 1/4 damage from fighting and grass attacks is quite good in itself, especially when you need to switch in on a powerful STAB fighting attack and threaten with your SE STAB Flying. And, of course, it's not weak to Flying.
Now, when I think about the idea of regaining momentum, I think of Crobat in DPP UU. Crobat was amazingly good at it--so good he wound up in BL. Come in on one of your great resistances, kill sweepers with STAB Brave Bird, Taunt walls, U-turn out to whatever you want, and Roost back your HP so you can do it all again. In addition to being a testament to the fact that Poison/Flying can work quite well, it's also a testament to the fact that SR weak, in and of itself, is not that big of a deal. Eliminating CaP1's rock weakness may be a good idea, but we shouldn't do it just because of rocks.