Exactly, Thameslink South is a mess on the map.
In truth all the SE routes should be routed into Elephant and Castle and one of the Sydenham metro routes should’ve simply been extended into the core since they couldn’t add extra routes through windmill junction then just have GTR take over the Coulsdon Town service via Sydenham up to Luton, it’s that simple
It's not that simple. Consider Bricklayers Arms Jn, which would have 16tph core-bound (lets say 12tph from Windmill Bridge, 4tph from the Sydenham slows). Crossing through all of this would be all the stuff coming off the Fasts going into the terminal platforms (2tph each from Uckfield, Coastways, Epsom and Cat/Tat).
That's 24tph one at a time over this point - nothing can be done in parallel. So Thameslinks need to be 5 minutes apart in order to get something else into Low level between them (repeating 2.5/2.5/5/2.5/2.5/5, etc.). But then need to transition to alternate 3/3.5 minute headways to proceed through London Bridge. Although it might work on paper in this area you've just created a timetabling hardpoint around which the whole of the rest of Sussex must be driven.
Sending 4tph off into Kent at least creates some timetable 'flex' to suit pathing. Sending them to the Sydenham slows forces everything into a rigid 24tph pattern.