I fixed it, and you will not believe what the cause of the issue was
I bled the system again (twice), which didn’t work. Then I gave my engineering skills a shot, by blocking part of the grille so the wind couldn’t get to the engineblock (as much), which (in theory) should result in the engine keeping more of its heat.
Which I thought looked pretty decent, but didn’t work. The engine seemed to get up to temperature slightly quicker while stationary, but it would still drop to around 60°C when I started driving.
I didn’t feel like going through the whole process of draining/filling/bleeding the entire cooling system again, but in the end, there was no other option than to check/change the thermostat again. I’d already checked the temp gauge and sensor by swapping gauge clusters and using thermal imaging, but both of them appeared to be working just fine.
@Roffelkut came by and gave me an OEM thermostat (which was very sweet of him), and so, last Sunday, I went to work with the help of a friend.
This was taken at 17:39 / 5:39pm
I drove home and well… nothing changed. Still running cold. By this time I felt like I wasn’t going to be able to fix it myself, and wanted to send the car off to a mechanic for diagnosis. That was a real bummer, since I bought this car so I can work on the Blyatsu without having to worry about any issues while driving. That night I dove back into the workshop manuals and Amayama to go over the entire cooling system once again, and then I noticed something.
In the middle of this image, you can see the inlet hose running directly from the radiator to the thermostat housing.
When I saw this, I thought “Huh, that’s weird. Then why was it so difficult to pull off the coolant hose?”. It was difficult because the T-piece (for the heater and idle control) made it hard to manoeuvre the small piece of hose attached to the thermostat housing.
Wait, a T-piece on the inlet trajectory? That doesn’t make any sen- oh dear lord. The hoses were fitted the wrong way. I ran outside to see if this was true, and waddayaknow…
This is how the hoses were fitted on the Sirion
This is how they should be fitted
I was weirdly excited by the fact that they were fitted the wrong way 
.
On Tuesday I drained/filled/bled the system again, and…
Taddaaaa, still up to temp, even after driving!
I was a little disappointed tho. In the fact that someone took off the hoses (wasn’t me btw), but wasn’t able to read a freaking temp gauge, and in myself for spending so much time fixating on the thermostat and overlooking something in-your-face as coolant hoses. On the other hand, if it was obvious, someone would’ve mentioned it, and it’s things like these from which you can learn and gain experience.
Now I can finally get back to work on the Blyatsu…