Snatch Single Leg Takedown / Street Finish
An alternative setup is to hit him with L hand and forehead, and snatch up the leg, but then immediately back up and pass the leg from the R hand to the left then snatch it up under the L armpit.
(available on Ebay, apparently.)
From here you have several attacking options.
From the back strap, he may decide to post up with both hands to get posture. Let him do this and come up with him. As he comes up, post behind you on your L hand. Release the guard, let your L leg go limp and push with your R leg to hip bump / sitting rollover sweep him to your L. Your R hand should move from inside your R knee to grabbing the outside of your L thigh, thus trapping his R arm. The thigh grab should be higher percentage at trapping the R arm than trying to grab or overhook his R arm or elbow as you sweep.
If he grabs your head at the beginning and tries to get a head/arm choke, you may need to back out and instead go to shoulder control on his R arm, with various strategies from here.
But in the long run
There's still time to change the road you're on
Gi version of Sitting Rollover sweep
Grab his L sleeve with your L hand. Pull him in and grab the back of his head with your R hand, pulling it down to your chest. When he pushes back up to get posture, sit up, post on your R hand and drive his L hand out to your R, Bump him with your hip and sweep him to your R.
He should feel reasonable comfortable because he feels able to post with his L hand to stop the sweep. As he goes over and begins to post, pull his L hand in toward your R hip so he is unable to post. You can now complete the sweep.
On the flip side, if you are on the bottom with the Z guard, do not the leg that is between his get caught too deep. It should be reasonably shallow, so it can't get pinched between the top guy's legs, and so you can employ it as a hook if the opportunity arises.