An update on the power readings: I was right, the Tacx WAS reading high. Compared to the calibrated readings on the normally-pretty-accurate Stages crank, the numbers the Tacx was feeding Zwift were 10-15% higher, which wasn’t physically possible.
However, I downloaded the “Tacx Utility” app for my iPhone and ran that, which updated the firmware and then calibrated the unit (it needs a LOT of pressure between the roller and the tyre!) and the numbers started looking more normal. The Stages and the Tacx were usually within 1-3W of each other, which was quite reassuring.
…of course, it meant that my “new FTP” from my first ride has to be discounted, because it was erroneously measured, but at least it gives me something to train for!
The cadence issue was also fixed by using the Stages as the cadence sensor, although that’s highlighted a separate issue where occasionally it will only register half my cadence (thus half my power). Time for a new battery and a firmware update check… all teething troubles, I’m looking forward to getting some more proper training in!
Also, I changed my reality setting to 100% because a) I’m training to ride in the real world and b) I’ve got all those gears on my bike, I should probably use them! I don’t think it gives me any particular advantage or disadvantage compared to other riders except having to change gears more often – Zwift say that the power output is what moves the rider.