You have to do some homework ;-) PCI bus powermanagment (ASPM) to autoIs this still the case for you, idling at 40°C? I've also got one of those with N5105 and it's idling at about 59°C or so, but it takes a while (maybe 20-30 mins) to get there, I figure that's when the heatsink gets saturated. Ambient temp is not warm either, it's about 19-20°C right now. I've been monitoring it over several days. Not really having any issues, but it's curious how yours idles nearly 20°C less. The thermal testing I did with Windows 11 pointed to the cooler being adequate enough.
Do you happen to have a way to monitor power consumption at the plug? I'm at about 13-14W idling, also Opnsense. Pretty much the same as when I tested with Windows 11 (on Win only 1 NIC was active).
Insert in your OS opnSense the C3 state. If it wont drop to 30 degrees there is a physical issue with your cooling.