Dan has looked into this extensively and published a few articles on it.
https://www.slowtwitch.com/...ift_Sensor_7614.html.
To date there is no treadmill that is truly smart in terms of allowing 2 way communication with Zwift. This limits the real-world value of a 'smart' treadmill because treadmills are very imprecise in how they measure and report speed. As a result having a treadmill send data directly to Zwift may be a clean solution but its a its not an accurate or precise solution. Foot pods, like the Stryd, should in theory be better but that relies on them being carefully calibrated. Personally if its your treadmill I would invest in something like the RUNN. Its accurate, reliable and shouldn't need constant re-calibration. This can be fit onto any treadmill which opens up the market in that respect.
The other advantage of using a Runn or a footpod is that they communicate via a standard protocol. This makes them much more future proof than relying on a treadmill. Treadmill companies have yet to adopt a standard protocol for transmitting data so companies are doing there own things. This makes then susceptible to rapidly becoming dumb mills as technology moves on. A decent treadmill should last many years of at home use and the internal software is almost certainly going to become obsolete before the hardware breaks.