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swap the thermal pads to thermal putty to get better compression
what i did when i reapplied LM to my zeph was apply the putty, then attach the heatsink and push it down to squish the putty in place, then screw it in and push down more.
then i unscrewed the lot and applied the LM and PTM then reapply heatsink
so i could get proper contact
The last update: the problem is kinda solved.
I opened up the laptop yesterday to see if the contact between GPU and heatsink is good, and the contact was bad once again:
My guess was that because the surface of heatsink that contacts VRAM (silver plate with a lot of big blue piles of putty on the photo above) is not leveled with the surface which contacts GPU itself and is closer to motherboard when installed, the GPU had no proper contact with heatsink on that side of crystal because that side of heatsink kind of 'hanged' in the air because of the thickness of solid thermal pads (last year) and the amount of thermal putty I put on VRAM a few weeks ago.
So what I did is took some of the excess thermal putty off of VRAM chips surface, spread the rest evenly so the layer wouldn't be > 0.5 mm (because I know for sure that 0.5 mm and thicker prevents GPU from contacting the heatsink properly as I applied 0.5 mm solid thermal pads last year) after the final pressure is applied, then put some fresh thermal paste on GPU and CPU, put the heatsink in its place without screwing it in and applied gentle pressure, then lifted it up to see if the contact with GPU and CPU is good, which it was. So I finally put the laptop together and ran some tests.
As a result, the temperatures of GPU dropped 5-6° C after this 'procedure'. The new temperatures are in somewhat comfort zone for me personally, although they are not the same as they were when the laptop was new and untouched.
I'm sure that if I experimented with the thickness of thermal putty layer more, I could get these temperatures even lower, not to mention the possibility of applying liquid metal (which I won't do, the risk is not worth it for me) and using different (better) thermal paste in GPU and CPU to help with a heat spread even more.
tl;dr: the problem was in bad GPU-to-heatsink contact. A very low amount of thermal putty should be applied to VRAM chips and VRM elements just so that after installing its thickness would be ~ 0.2-0.3 mm. That way the heatsink would sit properly and all surfaces would have a great contact.
Thanks for all the help and advices, guys.