As others have said you need to heat-sink it. If the inside of the case is getting too hot as well then you need to try to heatsink it to the case itself since that's meant to act as a heatsink. Without airflow you're going to have issues no matter what.

In general to keep NVMe heat low you'd want to choose gen3-4 if possible as those run cooler than gen5. You'd want to choose one with DRAM built in rather than a pseudo-SLC cache which will heat up the drive dramatically when it's hit. And you want a heatsink on the drive itself. Obviously you already have the drive but something to keep in mind for future reference.

There's a lot of options for heatsinking including large aluminum heatsinks you can buy with thermal tape that can attach to the outside of cases. The problem with that in your particular instance is your case has fins so the contact area between such an external heatsink and your case would be bad and you'd need to jam thermal putty in there and probably still get mediocre performance.

There are mini-peltier pad elements you can buy cheap bags of which you hook up to an appropriate DC or AC power supply which you could attach to the outside of this case as well to directly cool it by drawing the heat out and apply cold. You can find pre-built ones with fans attached and USB power supplies if you just search peltier cooler USB.

But these do use electricity to run so they reduce the efficiency of your mini-PC and you still run into the problem of the fins on the outside creating very poor heat conductive contact.

Generally you want to avoid those kinds of passive cooling PC cases. Even the well built ones suffer heat issues for various components. They're really meant for low end projects like running a display in a mall that shows a map and does basic tasks on a mini OS. Not for running a PC you're regularly reading/writing files from.

One last thing. Unless that's a reversed fan (unlikely) you really want it blowing air on the case not trying to pull air away as you have it. It's much more efficient that way in most cases. There's not enough clearance for it to be a very effective puller fan. So in my opinion try reversing the direction you have that fan mounted.