What do you need that your smartwatch doesn't already give you? Power might be valid and useful. However speed and distance are not. You aren't going anywhere on a trainer. Just my opinion.

Most of the Garmin watches can be paired with various cycling sensors such as power meters, cadence sensor and if you must a wheel sensor.

For the rare times I do ride a trainer, I find that putting the TV or computer on a bicycle race like cyclocross takes my mind off the drudgery of indoor riding. You can find them on YouTube and they last about an hour.