Smart keeping DE outside the playside tackle(assuming there is no TE). That's exactly what you want to do.
Pistol can be tough because technically they can run anywhere even oz. That's one of the reason a couple years ago I became more of a prostyle undercenter guy. I like the ability to run anywhere without tipping off the defense. The pistol does this with the added ability to run iz read game. TBH if I was a real coach I'd probably run a pistol type offense.
But yea pistol can be tough especially sets with no slot to cheat off of. However, the essence of strategy is maximissing your strength and limiting your weaknesses.
If you are in single high defense the strength of your defense in the running game (numerically) will between the tackles. That's where your want to force the ball. Are you going to stop them for no gain every play? No, but over the course of the game if you are force the run into your numbers on a consistent basis you should have the advantage. Especially since I don't feel the pistol is blocked really well consistently in this game.
Make sure you watching your opponent and gaining information and tendencies. What is the initial track(the path the rb should be taking based on playart) of the of rb? If it A gap(s) the put people there and take away his lineman leverage against you.
Long story short I think your doing the right thing. Your taking away his ability to maximize the strength of the play by taking away his aiming point gaps, and forcing him into your numbers. That's pretty much the best you can do. It's hard to stop the zone read in this game.
(For my basketball fans)It's like playing steph curry the best strategy is to switch all screens and force him to take tough off the dribble shots. However, alot of the times he still kills you with that. Sux but, it doesn't change the fact that its still the best strategy against him. Defensively you want to take away you opponents strengths and make him beat you with his weaknesses over the course of an entire game (even if its a good weakness
)