Just some additional insight coming from a 2013 with 99600 on the clock to date...
I do all of my oil changes at 5k - recommended interval is 10k. It has always consumed a small amount of oil, but nothing excessive. MAYBE half a qt every 5k.
I've changed the manual gearbox fluid twice with Redline MTL, shifts perfect. Clutch is in great shape from what I can tell, and I don't drive it gently.
I am on my original water pump and manifold, never any problems there. I did change the PCV valve as preventative maintenance around 70k at the same time I had a carbon cleaning done. I got a progressively roughly rougher idle over some time and it eventually started throwing codes. Carbon cleaning made it feel new again.
I've never used VAGCOM and while I have nothing bad to say about it, you can also just get a $20 VW/Audi code reader on Amazon like I have for reading any potential codes and save some coin.
The only part I have had to replace to date was my trunk release mechanism, and its 10 minute job. My experience has been to go with OEM parts on this car when replacing something...I bought a cheaper PCV and the seal leaked; I bought a cheaper hatch mechanism and it failed 30 days later, but the OEM parts have been perfect.
You can also cheaply add Apple Carplay to this generation GTI by purching the RCD330+ headunit on ebay out of Chinese vehicles. I think I paid $200 shipped and it was plug and play for me. Its an actual VW part, the plastic and back lighting matches perfectly. You would never know it didn't come with the car.
I've never owned a single vehicle as long as my GTI before and it says a lot. I enjoy driving it everytime and it runs as well as it did the day I drove it off the lot.