@TheNarc Thanks for all the information. Yeah, I also got a fan, this one, a bit larger, and with three different speed settings. But only one. Was also planning to attach it with cable-ties if needed but I still would prefer to make do with just passive cooling. My memtest86+ experiement with just the added SODIMM heatsinks unfortunately failed as well after about 1.5 hours. Again the RAM was too hot to touch.
Currently though I have 10.5 days of uptime with no crashes with no active cooling. What I changed is I left the SODIMM heatsinks on (even though they probably change next to nothing), I changed the TCC offset to 40 (which causes the CPU to throttle at temps above 65C), and I lowered the speedstep setting from two steps toward performance to two steps toward energy effiency.
The box is probably not 100% stable still, it could probably crash during prolonged stress-testing, but hopefully during normal usage it will be stable enough for what I want to use it for.
As for my NVMe drive I got one with the box and it seems to be some cheap chinese brand: BKKJ nvme 128G. It seems the current temperature it reports through SMART-data is broken (it always says 40C). But it does have other historical thermal information which is probably correct:
Warning Comp. Temp. Threshold: 83 Celsius
Critical Comp. Temp. Threshold: 85 Celsius
Warning Comp. Temperature Time: 12
Critical Comp. Temperature Time: 1
Thermal Temp. 1 Transition Count: 51
Thermal Temp. 2 Transition Count: 1
Thermal Temp. 1 Total Time: 5488
Thermal Temp. 2 Total Time: 12
I'm not sure what the unit is for the time but obviously it thinks it has spent some amount of time above 83C (warning temp) and a small amount of time over 85C (critical temp). Probably the SODIMM RAM increases to similar temps when the ambient temp in the box becomes really high. For now I think I'll only use the fan if the box keeps crashing during my normal usage or maybe during critical heavy operations such as full system upgrades. If I do use the fan performance does increase a bit as the CPU doesn't have to limit itself because of thermals but for my use I don't really need every last bit of the possible performance.