STEPS senses torque/cadence at the crank to give proportional assistance, and has three overall power levels (four if you count 'OFF'), selectable via the excellent DI2-style thumb buttons. It certainly doesn't have to provide assistance all the time.
Barakta's Falco system does something similar (sensing torque/cadence at the hub and multiplying that by one of 5 power levels), but since we've tweaked the programming to better suit her needs (basically to minimise the assistance while cruising, allowing for her fairly mashy pedalling style), it needs quite a lot of torque before it actually gives you a significant power boost. I suspect you couldn't deliver enough power uphill in a high enough gear to make it kick in (there's a programmable minimum speed threshold too, which IIRC we turned up to ensure the motor would stay off during low-speed Silly Sustrans Gate manoeuvres). I'm sure it could be programmed to be cycleman-friendly, but that means connecting a laptop and the horrible Falco software and spending some time doing laps of a car park changing various parameters.
Anyway, the important thing is that it's perfectly normal to operate these things with the system powered up (so the console is alive and acting as a cycle computer, lighting power[1] is available etc) and the assistance level at zero - changing assistance level is something you'd do to suit the conditions, like you'd change gear. So you can certainly trundle under your own steam and only turn the assistance on for climbs.
You won't get a throttle-only system any more, because the rules that made that road-legal were deprecated by the harmonised EU regulations, which require a crank-rotation sensor of some kind (so that the power assistance always stops when you stop pedalling, regardless of any other controls). A throttle to control the power (rather than set levels) with a basic yes/no crank rotation sensor is allowed, but that tends to be the domain of the cheap Chinese kits.
One thought occurs: If you think your balance would be up to it (and bearing in mind that keeping up speed isn't a problem with a motor), it might be worth trying some upwrongs with the relevant motor systems, to get a feel for their differences. Well-behaved sit-up-and-beg city bikes are the natural home of electric assistance, and an hour playing with a sympathetic dealer's (I'm thinking 50 Cycles or Cycleheaven or some such) demo models would be very enlightening.
[1] My friend did a test with their STEPS ebike: When the battery got flat enough that the system shut down under load, there was enough power left to run the DI2 gears and lights for about 3 more hours before they got bored and went to bed. You can reasonably expect an ebike battery to be able to power the lights and electronics continuously for a couple of weeks if you don't actually use the motor, so there's no need to be paranoid about wasting battery by having the system switched on.
