Hard to reach opersting temps (both coolant and oil)

Car: 2015 Octavia Scout, manual gearbox, 2.0 TDI (CRMB), 230000km

Hi,

My octavia has an issue. It takes a really long time to reach operating temps. Yesterday driving at 90 km/h (+5 min city traffic first) and 10°C outside, it took me almost 15 min before I had reached 90°C coolant temp and ≈ 75°C oil temp. Driving speeds below 50km/h I can barely get the needle to move.

Oil temp also has a hard time to go above 80-85°C going 80-90km/h, but driving in highway speeds I can reach 90-110°C depending on outside temp.

Also if I engine brake and roll down a long hill, the coolant temp drops ≈ 5-10°C. This happens even in the summer, with temps around 20-25°C.

So, my main hypothesis is a broken coolant thermostat, since I have no error codes and the fault has just appeared without leaks, noises or any other symptom. Water pump was swapped last year, hopefully it's not that. One thing i'm not sure about is if I have the easily accessable DSG thermostat under the intake, even though I have a manual gearbox?

What do you people think, should I try having a mechanic swap the thermostat (the main one that's hard to reach)? My guess is it will cost a couple hundred euro, so that's why I'm asking here first if it could be something else.

Edited 33 minutes ago33 min by Haxel