Welwyn to Sevenoaks presumably isn’t that difficult to do, and as you say has the practical outcome of freeing up a platform at Blackfriars - albeit only in the peak.
Maidstone to Cambridge is the fly in the ointment. If they implement Welwyn to Sevenoaks on its own it doesn’t help free up a platform at King’s Cross, and gives a messy situation of having the peak Welwyn service awkwardly split between core and King’s Cross, as well as Knebworth and Welwyn North having no core service at all. However on the plus side the long turnaround times at King’s Cross and Cambridge do have a positive effect on performance for what would otherwise be a highly fragile setup - on top of the existing fragility brought about by the Brighton and Horsham GN services.
It would probably make sense to start Welwyn to Sevenoaks, and leave Cambridge to Kings Cross as is, perhaps tweaking things to run off one platform at Kings Cross. I’m not fully certain how readily simple the latter is to achieve - outwardly it would be a simple case of just holding the incoming service at the home signal for a couple of minutes, but I’m not sure if that blocks the Thameslink junction, which if so would be undesirable.
There’s also the longer term quandry that with Kings Cross being remodelled, the final layout ideally separates out the Thameslink services from everything else, whereas if the Cambridge services are left using the high-numbered platforms they will need to make a conflicting move at the flat Thameslink junction - not sure how the final layout will handle this, although 2tph out of 8tph shouldn’t be too much of an ask.