After 15 disconnects in/after lap 1 within 4 months I've finally found a workaround.
This same problem also exists in GT Legends!
Pretty easy to explain, after lap 1 when crossing the timing line my lap time is not recorded, I drop down the standings as other drivers complete their 1st lap and a few seconds later I disconnect from the server properly.
Problem approaches, findings etc.
An important requirement to fix any issue is obviously being able to reproduce the issue, which at first I couldn't do with driving race sessions on servers all by myself, so I had to resort to our events to try out different things.
I've tried out many different settings, from BIOS over network configs + physical network changes and antivirus apps to settings within GTR2 itself - no success.
A new Windows 10 install with just GTR2 and nothing else on it, a different PC - but still no success. So most likely the issue has to do with my internet service provider.
At some point I noticed that disconnects seemed far more likely on long tracks so I looked at the lap time of the opening lap of the leader and myself for all my 48 races: Apparent was that the longer the lap time the more likely it was for me to disconnect (chart attached). The critical variable seems to be the laptime of the leader as found out by testing with Shawn.
With this knowledge I was able to reproduce the issue by myself as I could disconnect even on the shortest tracks by waiting/driving slowly to produce a lap time somewhere around 2:20 upwards. So now I could conduct a lot more tests in a short amount of time and also found an old forum post by Steini, who could solve his disconnects around the Nordschleife by moving and subsequently changing his ISP, which supported my thesis about it being an ISP issue.
I disconnected the ethernet cable from my PC and instead bought mobile data for my smartphone and let my PC run over that through USB-Tethering and don't experience disconnects anymore, no matter how long the lap may be.
My connection quality over mobile phone data (4G) is excellent and stable, no problems with high ping or lags.
FYI: For one online race over 1h plus 20min of Qualifying and 10min Practice (so 90min overall) GTR2 seems to use between 125-300MB depending on grid size, track and cars.