Why not buy a decent conversion kit, maybe not as much flair as a factory ebike but it will do the job and if you go off it you can put back to standard easily enough
Too many variables to answer this accurately. You get out of it what you put in. I would say I’m fitter because I use it more and for general commuting tend mostly to ride in lowest setting or off altogether. I’ve also had some of my highest heart rates going for it as hard as I can on full power.
emtb is best of two worlds if your feeling fit and up for it ride with motor off, it’s a better workout than a clockwork bike. From there use whatever power level suits your mood or how energetic your feeling.