I saw interesting article and chart the other day about 350KW chargers. The big power dump happens within the first 30% of battery capacity. At about 50% of capacity, charging curves start to match up with the slower 120KW chargers.
That's entirely correct. When charging an EV with a 350kW charger, there are two things that keep you from having the full 350kW effect during the entire charge.
The first is that all EV's tell the charger to quit charging more than 50kW beyond 80% charge. This is to protect the battery's longevity.
The other is that batteries soak in charge at a less efficient rate as they get more and more charged. So when I start charging my EV at a 350kW charger, if I come in at 20% charged it'll drink in the charge with almost no loss in efficiency. But in the 10 to 15 minutes it takes to get to 80% the efficiency it accepts the charge is a little less, then a little less, etc. So even if the charger is showing a full 330-350kW the entire time, in reality the %increase slows the higher it gets.
That's me as a nerd looking at it. For all practical purposes as an EV owner on a trip you don't care about that as long as you can get charged up to 80% in 10-15 minutes and get a couple of hundred miles out of it (since my wife likes to stop every 200 miles or so to stretch her legs anyway).
When I say that I get 3.0 to 3.1 miles/kWh out of mine, that's charging from home and assuming a 10% loss in efficiency. Part of that is the conversion from AC to DC, part of that is the EV is usually charged 50% to 80% so it accepts a charge at a less efficient manner even if I'm charging at full speed (the efficiency loss you pointed out), and part of that is because I usually have it set to charge at a slow speed at home (a hair over 5kW), which is even less efficient when converting AC to DC. I charge at that slow speed to reduce the odds that our overall load from our house doesn't exceed 18kW (in which case our solar inverters would pull the excess from the grid because their max continuous throughput is 18kW in converting DC power from solar and/or batteries to the house panel). If I charged at full speed like most EV owners, I'd probably get 3.2 to 3.3 miles/kWh.