First Rule of home automation: “the model number matters.” You just need to check the exact specs of any individual models you are considering. In general, Zwave plus devices should Have smaller radio boxes than Z wave classic, but it’s up to each manufacturer to design each model the way they want.
In some cases, you can put a collar on the switch box that then lifts the cover plate up a 1/4” or so and get more room that way. These are intended for situations where people are adding tile for kitchen backsplash or something like that, but they work fine for smart switches as well. These are also called “extender rings“ at places like Home Depot even though they aren’t round.
BTW, leviton is one of several companies that make the nicer looking ones in this category. I’m surprised your electrical expert wasn’t aware of them.