Well the since the Xiaomi / Aqara sensors send reports based on changes in temperature or humidity it is still possible to use them as part of an automated bathroom vent fan setup. I haven’t done this myself, but I have read of other people doing this.
Regardless of whether you use a Xiaomi / Aqara sensor or another humidity sensor, you will need to either set up an already included SmartApp or install a community-created one for the automation side of the equation. For this use case, I’d look at the choices available in the Marketplace section of the SmartThings Classic mobile app, in the SmartApps tab, perhaps under the Climate Control heading. There’s a SmartApp called Humidity Alert! which might do the trick:
As for other temperature / humidity sensors which may report based on a time interval, there are so many choices available at a wide variety of prices I’m afraid I cannot give a recommendation here. Perhaps search the SmartThings Community Forums here for a thread in which someone has asked for the same recommendation?
Hopefully you won’t ever have to modify code, because what you are looking for are SmartApps to help you create automations. The Device Handler (or “DTH” or “DH”) code for Xiaomi / Aqara devices that are linked to in this thread are just simply for making the devices accessible to SmartThings to use in conjunction with SmartApps. Thing of Device Handlers as device drivers that you need to install on your computer in order for it to recognize those devices when you plug them in, and SmartApps as the Applications that allow you to do things with the devices.
As for information on where yo get started, there is A LOT available on this community forum, so it can be overwhelming. Here are a a couple suggestions to get you going:
and also:
