I wonder if the 50mph threshold ought to allow for some local variation. For example, the Santa Clara County (California) expressway system is tagged as highway=trunk, and I think this tagging makes sense. (These are Central Expressway, Foothill Expressway, Lawrence Expressway, Montagu Expressway, San Tomas Expressway, Southwest Expressway, East/West Capitol Expressway, Almaden Expressway, and maybe one or two others.) These expressways have limited at-grade intersections (and the ones that exist are a mix of ramps and lights); they’re clearly more important than a number of the highway=primary surrounding them, but also less important than the actual freeways (which are highway=motorway). Yet in many cases the speed limits are 45mph, which doesn’t quite fit your rules, but I think the trunk tagging makes sense.