On gravel or road racers, I think a cadence sensor is essential. I also have a cadence sensor on my MTB. It happens less often with the MTB that I notice it's better to ride in an easier gear thanks to this sensor. The watt sensors are currently far too expensive for me, blatantly expensive stuff! I've already put such posh-posh pedals in my shopping basket several times and then thrown them out again. Unfortunately, I can't have a qualified opinion on this.

The one has nothing to do with the other. The cadence sensor is included in the bundle and is not expensive. Watt measurement is expensive and not included in the bundle.
I can't say whether it makes sense. Cadence sensor only ideal for the road approx. 80-100 revolutions per minute.

The question is more of a fundamental nature. The Garmin Edge 1040 cannot measure power and/or cadence on its own. Cadence works with the magnetic sensor included in the bundle. For power measurement (incl. cadence), the Garmin Rally XC100 or XC200 (both sides) pedals for the MTB are required. Then the watt and cadence measurement works wonderfully and is of great use to me to be able to compare my performances over the years. In addition, the XC200 system also measures the power distribution left/right and where you are on the pedal. This is all very good information and helps me - whether MTB, gravel or road bike - to optimise my pedalling.
The solution is not cheap, but gives excellent clues to compare yourself or with others and to know where you stand.