Not as much as an 18:00 departure would be carrying with the "9 to 5" workers
In my experienced the busiest departures from both Glasgow terminals tend to be 16:30 to 17:30, usually due to the number of shoppers, uni and college students, in addition to office workers (whose number is diminishing now, but I’d say the core times have generally been consistent since pre-COVID).
You could probably consider the ‘shoulder peak’ to be from 15:30 to 18:30, but I’d say the core is pretty much as above. Hence additional peak departures from Glasgow city centre tend to be in that 16:30 to 17:30 window.
The exception being the E&G, although those are arguably as much about Edinburgh as Glasgow (and I believe the 15 min frequency will be extended by an hour or so in both peaks next year).
As for the express motif, it is partly a throughback to old publicity which was for the Glasgow/Edinburgh/Aberdeen triangle, and as others have mentioned, aimed at differentiating from slower Glasgow to Edinburgh services. ScotRail may wish to alter this when the class 222 units arrive, as these are not likely to have the Inter7City branding.
Personally I think a bespoke fleet of trains for the main Edinburgh to Queen Street would be preferable. It’s hardly just another suburban service. With class 170s operating it had more comfortable seating that now and had a trolley service, these should really be minimum expectations for such a route.
Coupled with full electrification of the borders line (not happening but arguably should be) would the enable the spare class 385s to be used here instead, and could be inter worked with the North Berwick line.
