i strongly recommend the "cadence" cycling app for iPhone (and i'm sure the android version is excellent.) rideWithGPS is better for routing and nav but cadence has the best displays for keeping tabs on heart rate, speed, cadence, elevation, etc etc. super customizable, tons of graphics options, very easy on the battery particularly with a modern (OLED) phone and black background.
get yourself a chest strap heart rate monitor (most all have bluetooth and ANT+, you want bluetooth for sure) and then you'll have speed, heart rate, distance etc on your iPhone. quad lock is the most robust way to attach the phone to your bars. get a cheap bluetooth cadence sensor and you can have cadence too, which isn't all that useful IMO but good to have. if you really want to know how hard you're working, you'll need a power meter of some sort, which is probably over the top for your application. which zen bike do you have?
if you'd rather have a standalone device, lots of good options out there but most of the garmins (530, 830), or the wahoo bolt v2 are more than capable of what you're looking for. beware that while most all ebikes support bluetooth for communication of metrics to their own head units or apps, they do NOT typically support the standard bluetooth protocols that a cycling computer or phone app uses. so you'd need to use ANT+. no idea if zen supports that, but most of the big ebike motor/controller manufacturers do.
