Its indeed unfortunate that Sheppy has lost this direct link to London Victoria.
Personally, though it might be a bit far and could damage the NKML/Chatham mainline timetables, I always thought Sheerness would have been a good terminus for the Thameslink service as well as Rainham. Still, I can imagine it's too long for many of the intermediate stations, so it's more or less pointless as an idea.
The 17:51 service is normally 10-car, which is far too long for the branch. It might be alright if it could detach at least 4 cars at Gillingham, but that would consume resources and platform time.
I mean, there is sometimes coach splits at Faversham anyway. I can't remember the exact number of coaches, but some continue towards Dover Priory via Canterbury East, and some continue onto Ramsgate. Sure, it would need more staff, but providing the logistics were worked out in terms of staffing and timing, it doesn't seem impossible?
As for restoring the half hourly service off peak, I would assume that would depend on whether any 375/3s are spare from mainline working or the Medway Valley.
Perhaps, though to be honest, the half hourly Medway valley service has been the best thing that has happened to that line in years, apart from the peak time Javelin service (I wish it was permanent throughout the day

). So hopefully instead, it would come from splitting a Dover priory service further at Faversham instead.