I have noticed two issues with the EG and GTW services you mention when coming home from school that have me really cheesed off.
Sometimes part of my journey home involves me changing at Norwood Junction. I'll then usually get a train to East Croydon to catch a connection. The Gatwick Airport trains from Norwood run at 02 and 32 past the hour, except for when I'm there for some reason. I get to Norwood long after the 15:32 has departed, but for some reason there is no 16:02 departure, the next one is at 16:32! This isn't a cancellation, it'a just a service doesn't exist at that time. I then have to get the 16:09 Coulsdon Town service which is of little use to me as it means i just miss my connection at East Croydon and then have a 25 minute wait. Why doesn't this service run? Is it an amendment due to the failure of the new timetable, or has that always been the case since the timetable started?
The second thing is rush hour East Grinstead services from London Bridge. I've been at East Croydon whilst noticing this. Now obviously the usual EG departures are XX10, XX40 from Victoria. Previously in the evening peak this was supplemented by a 1730, a 1758, an 1830 and a 1900 (or somewhere around that time). Since the timetable change these extra services now start earlier, the first one being 1655 and then 1725, then 1755, however there is no 1825, which is more or less at the height of the peak. The next one is 1855. Is this a similar subtle amendment that could have a big effect on someone's commute?
This is all not to mention the 2 hour gap between thameslink trains leaving Brighton halfway through the day.
A further annoyance is at stations on the line between Carshalton and Mitcham Eastfields. There are 4 stopping trains per hour but they are separated by an infuriatingly unfair 4/26/4/26 minute gap. Whilst there has been no real service reduction it does mean that the frequency of stopping trains has effectively been cut to 2tph. However this is just a general fault I see with the timetable itself rather than the effects of it.
There is meant to be a 1602 from Norwood Junction to East Croydon but it is a 'planned' cancellation
http://www.realtimetrains.co.uk/train/W64470/2018/07/02/advanced
Similarly the 1825 from East Croydon to East Grinstead is a 'planned' cancellation
http://www.realtimetrains.co.uk/train/W64106/2018/07/02/advanced
In the same way, there are many planned cancellations on all routes - this is what leads to two hour gaps out of Brighton.
On the spacing of East Grinstead trains at East Croydon, it looks like xx10 for the Victoria and xx25 for the Thameslink trains is a good outcome with the Thameslink train following the Uckfield service, it is unfortunate that they can't get all the times to be perfectly spaced on every route.
With the way in which the Thameslink services have to fit in with other operators (including Great Northrn and Southern), it is difficult to see how you can cut Thameslink services and remain with a balanced timetable. For example, at Redhill, the two Thameslink service groups are Gatwick to Bedford and Horsham to Peterborough - you can't simply make one of these work by cutting out the other because different drivers are involved - there has been local suggestion about 2tph from Thameslink to Redhill running but this could easily end up being two trains running 10 minutes apart and nothing else in the hour.