Ahh, welcome to the data/metrics rabbit hole.
A phone handlebar mount and a cycling app is a good entry point. The Coros Dura is mid-range. The Garmin Edge 840, 850, or 10-series are high end. I don't recommend the cheaper Garmin 5-series because the lack of touchscreen is infuriating with the detailed Garmin UI.
Start recording activities to a cycling platform w your watch. Most would use Strava. RideWithGPS is another good free choice. Make sure to authorize Health/HR permissions so you also record HR.
You'll probably also want to watch metrics on your phone while riding & recording with your watch. This will prob require you to start recording with your phone too. Just make sure you don't double-upload. I have too many friends that record and upload to Strava with both their watch and their phone and screw up all their totals. Discard the phone one because it won't have HR.
For cadence, you’ll need a Bluetooth cadence sensor. Bluetooth will allow it to connect to your watch/phone or a cycling computer.
https://a.co/d/fUmrCkD
https://a.co/d/ekllQOH - This includes a speed sensor, which is unnecessary for road riding but good for MTB. The cadence alone is the same price
For watts, you’ll need a powermeter solution which will be >$500 and may require cycling shoes. Powermeter pedals are the easiest option, but def require shoes. A crank powermeter is the next best option that could be used with flat pedals - but it's new bike parts that can present fitment challenges for diff bikes.
If/when you buy a cycling computer, you'll record data only with that. You could download another app to broadcast your AppleWatch HR to the computer. Or you could use a diff heartrate monitor (HRM). Chest strap HRMs provide the best data, far better than any watch. Bicep straps are also good, getting more common, and can be more comfortable.