Finished it myself last night, with the exception of part of mission 7 and 8 it was way easier than SE4 I thought.
And now I see today that you can just lower the difficulty and bump it back up to authentic before exfiltrating and it will count.
I don't think I'd have enjoyed it as much on a lower difficulty though, authentic suited how I like to play. From what I've seen mopping up achievements it's an absolute cake walk on the easier settings.