Figured I'd write up a quick guide on the return line thermostat as well. This one is mean to "replace" (without having to remove) the original main thermostat.

I used this: 4029783001093 thermostat, and a hose clamp to secure it in the return line. Mine came with a pre existing weep hole. Although I don't think there's any chance it'll go anywhere. It's a very snug fit.

I saw another guide recommending a 3531650004034 thermostat and manually drilling a 2.5mm weep hole, but I would NOT recommend going with that one as it is much larger. I couldn't get it to fit at all.

The return line is easily accessible from the top. Most common seems to be removing the hose from the radiator and shoving the thermostat in there, with the "thin" end towards the engine.
I split the hose in two (see red marking), put the thermostat in the hose towards the engine and then spliced the two ends together with an alu tube around the same diameter as the hose just to get it closer to the engine. Although, it's perfectly fine to put it near the radiator if you don't feel like cutting the hose.



Here's the thermostat in. I used some soap to lubricate the sides of the thermostat before wiggling the thermostat in with the help of a pair of pliers and "working" the hose by hand.
Weep hole is towards the top of the hose.

Despite obviously needing a coolant flush, I just topped it off to give it a try, and it rose up to desired 88-92 degrees C after about 10 minutes of driving. I was getting up to around 50-55 prior to this, with a new EGR thermostat.

With the correct thermostat, the job only took about 30 minutes.