While it’s a simple fact that more speed = more drag = lower efficiency, I’d say that a motorway is a motorway.
If I may, respectfully, suggest, you might like to step back - maybe you’re overthinking this.
One of the (many) benefits of an EV is that you should arrive with nice low stress levels 🧘♂️
ACC is an aid to your journey - if it works for you fine. But, at first, it can be stressful wondering how it’s going to behave - even that it’s going to stop you when necessary. Whilst it absolutely will, it is indiscriminate in reading traffic around you on three- and four-lane. Don’t use it until you are comfortable with everything else.
Speed - follow the flow! So, you start off using a ‘bit’ more juice in your mind! But a) you haven’t got to worry about cost because you’ve already prepaid for the miles you drive and b) driving as you normally do and being able to anticipate slowing traffic means you won’t be burning up the electrons unnecessarily, or if you do you’ll part recoup them through regen.
You won’t achieve a stress-free journey if you set out thinking you shouldn’t go above XXmph but you have a 40tonner on your rear bumper for 100 miles.
Drive your own drive, in the knowledge you have a Plan A and Plan B for charging stops - you’ll end up being too conservative anyway.
Set the heating/AC for comfort. Play some nice soothing music. Enjoy the sunshine … and the thoughts of clean air your car is producing.
Let the electrons take care of themselves.