@f I don't wanna make assumptions but is there a way you could keep the NAS in a central home base location and self-host your data to a mobile endpoint via VPN tunnel?
@f sounds like a mini ITX solution is too bulky then. Not sure if they've got enough juice but I think there are some mini pcs that use their case as big dumb heatsink if that solves your heating issue. Best of luck with your project. I'm curious to hear what you end up with