You can’t beat Mark 3 coaches (as you mention) or the Mark 4 used on East Coast (though most people don’t think these are as good as Mark 3’s). One reason I dislike the Voyagers used by EMT and X-Country is the racket from having the engines under the floor boards. When train designers pick up on this (for long distance services) we may get a much better rail experience all round. But the 185’s I have used seem quite good in First though I didn’t get any snacks on a midweek service from Liverpool to Leeds – only a tea. But the First Class area seemed pleasant enough.