Checkout eMTB magazine and Downtown sister magazine both free online mags. Have review of cargo bikes.
I've carry 20-30kg on hardtail rear rack occasionally, not something I'd recommend on daily basis. Anymore than 20kg you need cargo bike. For middrive motors 60-80nm, I own new Bosch performance line 65nm and old CX 75nm both eat hills. Old small chainring motors can be dongled, latest ones with fullsize chainring can't.
Brakes, 2 pot Deore or MT4 are good starting point ideally 4pots if towing heavy load, easy and cheap to upgrade. I get +3000km out of standard shimano 11spd chain on CX drive dongle, running costs aren't that expensive if you do your own maintence and buy online. Expect to break a few spokes on rear wheel if it not cargo bikes, they come with overbuilt wheels.
FS is waste time unless its R&M Delite, Homage or Load. On most FS mtb rear rack and cargo doesn't benefit from rear suspension.
@TrevorB thanks for all that info, I really appreciate it.
In this 3 page discussion, I've had a ton of excellent help, and I'm now almost sure I'll need a cargo bike (and pay accordingly!). Frankly, I was hoping just to get an electric version of my last bike, a commuter hybrid, which survived my 120Kg plus a 40Kg backpack, so taking the weight off my back and putting it in a trailer on an equivalent ebike should be the obvious choice, right? Hmm, doesn't look like it. So cargo bike it will be.
Posts like yours have also made me realise that a high torque motor, like a Bosch CX, is going to be needed. That narrows down things a bit, as most of the CX bikes are very expensive, barring the Kona Electric UTE (I've found a 2019 version that isn't too dear). Standard chain, instead of belt drive, so my (very rusty!) bike mechanic skills are going to come in handy.
A second bike I'm considering, for a bit more than the Kona, is a Tern HSD S8i, mainly for the belt drive.
You mentioned "dongled". I went to a site that sells many dongles (including ones that supposedly deal with anti-dongle measures in gen 4 motors), but I can't see an obvious winner. Any suggestions?
Upgrading disc brakes isn't something I had been thinking about, thanks for the suggestion.
Does "FS" mean front shocks? If so, thanks for the advice, I wasn't too fussed about getting shocks (front or rear), but we do have some pretty dire potholes round about here in London, so maybe I'll rethink that. If "FS"
doesn't mean front shocks, what
does it mean?
Thanks again for all the info, very much appreciated. Now I'm off to those two magazines you mentioned, to read more about cargo bikes.