The intake manifold bolts were indeed quite loose, the worst was at about 10nm torque i would say. Had to tighten them by hand because lacking some parts to my torque wrench, but got them to about 20-30nm by hand.
Wen't for a test drive after all the stuff mentioned above and the result was the same as always with misfiring from startup which ended after 10 minutes of driving and the engine heating up. Today was a warm and dry day so the misfiring has evolved to the next level it seems. One thing i noted today was that after a hard 2nd gear pull i get a loud backfire when changing to 3rd. The car was also backfiring when downshifting and such. Not too much but it was noticeable when driving windows open.
After that i was fed up and decided to try to alter the fuel to air ratio manually from the dme. Changed the fuel quality switch from the motronic to position 3, which lowers the amount of fuel coming to the system by 3.9% it seems:
https://www.911chips.com/fuelsys.html
After that i got the first cold start in a month without misfires! The car run very nicely and didn't backfire noticeably after hard pulls, some backfiring still sometimes i.e when rolling downhill while engine braking with about 3-4k rpm.
This probably isn't the best long term solution, but proves that the engines air to fuel mixture is off by quite a bit. Could it be a faulty or dirty air flow sensor? I'm seriously running out of ideas, could the valve guides be still the cause?