Wild cadence readings

I’m relatively new to Zwift so I don’t have a good understanding of the tech stuff yet.
I’m using a Tacx Bushido & it was going well on my road bike, but I’ve recently put my old MTB on it, as I can pretty much leave it as a permanent set up.

During some workouts which cadence changes, my on screen cadence regularly fluctuates while my actual cadence hasn’t changed, which makes these workouts a bit of guess work.

I’m wondering if adding a Bluetooth enabled cadence sensor will help to get more accurate & reliable readings?
I’m running my Zwift through an iPad & android companion app.

Any recomendations?
Wahoo BlueSC?
JetBlack JBT-102?