Hi Deeby, I was in exactly the same boat as you when I bought my Gain in 2018. a bit overweight (still am), was fine riding on the flat and could cope with 20 or 25miles, but any more than that and even the slightest hint of a hill and that was me done and out he back of the group. The Gain answered all my needs. Riding on the lowest power is like riding with a gentle breeze behind you, sure you still have to pedal, but it's just easier and you roll over those little undulations. Real hills, no the Gain won't power you up, but it takes the sting out of them. You still need the lower gears and I still huff & puff, but I now get up the ones I used to get off and push. You mentioned cadence, the trick with the Gain is to keep it spinning. Use your gears, keep your legs turning and it assists. High gears and expect it to power you us and it'll bog down.
Looking at the longer term benefit of having a Gain, I guess I've riden more miles in the two & half years since i got my Gain than I had in the previous five put together. Despite what people say about electric bikes I am fitter than I was, I ride further, hill don't frighten me (as much), but maybe what more important of the lot is that I enjoy riding it. I go out more, go futher and with less trepidation of what if... I even do short Audaxs now on it which was pipe dream stuff only few years ago.
Buy one, you'll enjoy it.