I agree with what you say and here is my thoughts.
One of the issues with the KEB controller and I believe the KLS7275D controllers is they limit the current on initial take off
It might be, I heard more people saying that Kelly has more limited starting torque/current compared with Sabvoton. Specifically, I'm comparing Sabvoton 72150 and Kelly 7230S. Your controller is different but there is some relevance.
I've attached Sabvoton 72150 12'' to scooter. It it pulls ok from a standstill (scooter 105kg, hubmonster hub, 12inch rims, 72V battery ,reached 85km/h, max may be 100km/h, it must be fast winding hub).
9kV, 13kV, 15kV, 17kV and 19kV.
I assume less kv has better acceleration from stand still and better for slow speeds.
More kv has better dynamic after scooters reaches some speed and is more dynamic and efficient going fast.
More kv motor has less resistance, and more current it is capable to accept.
High kv & small rim & a lot amps & high voltage is holy grail for the hub. Nevertheless hubs will be always in bad gear whatever you opt unless you plan is to sping it a certain RPM, other than that you diviate from it's holy-grail-iness . So it's compromises everywhere.
It's hard to imagine how such big controller like KEB72121E can be terrible at startups. Maybe some other components like battery or motor is a limit itself. But again, I did not try KEB72121E controller my self. I can tell only about Sabvoton, it has good starting torque, for sure bigger start phase amps. Kellys is limiting current more depending RPMs compared with some other controllers. And it's good if you aim for components longevity.
With a lot phase current you can make any motor roll in fast; more kv motor, more heat it will make at startups, but more kv motor will be more dynamic and powerfull going fast, and overall it will accept more current and it will generate more heat.
You can track motor temp. Regarding max amps it's hard to tell when you will get in trouble. Unless you know motor specs and its limits beforehand; otherwise you push it to limits and see what hapends. Best indicator is raising temperature with properly installed KTY sensor.
If you buy qs motor you know it's limits. Manufature has recommendations. You don't go much beyond recommendations and you are good.
All few scooters I've tried they pull well from stand still. And I assume it is because of small 12'' / 13'' wheels.