My guess is that using a simple dsl modem, rather than the combo modem/router, is your best bet in tandem with the Airport Extreme (AE). Turning off the functionality of router in the combo model may not be possible, and if it is, it adds another a layer of unnecessary complexity.
A router is seen by your DSL modem as just a computer, so any modern router should work fine with most DSL modems.
One important question is whether the AE will serve you better than your current router. Your DSL line speed is probably less than even a 812g wireless connection, so a faster router will not remove that bottleneck.
And Verizon's DSL is not perfect. Slowdowns or freezes of Internet service are just as likely caused by Verizon as by the router or modem.