I haven’t fully converted my home to Z-Wave mostly because I have a bunch of dual and triple gang boxes that currently have Insteon in them. Insteon switches are 1.2 inches thick, so they fit OK, albeit tight. Are there any Z-Wave or Zigbee switches this slim?
Z wave tends to be bigger then zigbee but they’re both bulky. The newest generation of zwave plus is a bit smaller than the previous generation.
Your best bet is probably the Aeon micro relays in the Z wave plus model. They will fit in many places the others won’t, and can be used with any switch hardware you like as long as there is a neutral in the switch box.
They’re on the official “works with smartthings” list.