I think realistically it's only rail enthusiasts who would consider day-tripping a journey of that length, and having used the FW seats very, very few people use the northbound service as a day train, it's just too early. (It is used reasonably well in the other direction, though).
FWIW, the lack of a Sunday morning train from FW to Glasgow is I believe being remedied this winter (until now you had to use the morning Citylink bus, and it often books up well in advance). It will be interesting to see if that causes a reduction in sleeper passengers by making it possible to reach the South East by daytime train on a Sunday in winter, which wasn't previously possible.
Assuming availability, even a regular service could fulfil the role 19:00 FW to Glasgow service, with an early Sunday service probably as a repositioning run. Having 2 day services on the Sunday is very low.
When did the Sleeper last run on a Saturday? Now it's more popular than ever is it worth another shot?