Besides the NUC options, there are plenty of options in "tiny PC" form factors (for instance, https://www.servethehome.com/tag/tinyminimicro/ has reviews for several models). However, probably none of them will be fanless. If you really want fanless, you should look for industrial PC form factors, which often have to run for years in environment with lots of dust; there are several manufacturers once you start looking at it (for instance, http://www.gigaipc.com/).

NUCs aren’t fanless, the newer versions tend to be 25W and have very loud and whiny fans, and in fanless cases they tend to throttle, sadly.

I replaced one with an M1 mac mini and the mac mini is faster and near silent - I have literally never heard the fan over the room’s background noise, and I’ve thrashed the thing.

Intel NUC or any of the NUC clones produced by other PC manufacturers, like the Gigabyte Brix.

It’s larger than a Mac mini because it’s meant to be a NAS, but I’ve been running FreeNAS on an HP MicroServer for almost a decade now and it’s been mostly great.

They still make products in that line although I’m not very familiar with the newer offerings.

FYI, the Mac Mini M1 has a fan that never turns off. It's very quiet, but it is not 100% silent.

To be fair, it is really hard to get the fan to speed up and make more noise.

Protectli. They are fanless and use 7th or 8th gen Intel laptop processors. Uses regular laptop memory and mSATA/NVMe for storage. If you run OpenWRT on them they are blazing fast, but its a regular x86 box with an HDMI port so you can run a full GUI if you want.

Asus PN50 - has a good enough IGP too. Runs fairly silent (not dead silent mind you though), has decent power consumption profile and recent Linux kernels are stable on it.

I'm a fan of the Mellori-ITX: https://github.com/phkahler/mellori_ITX

It's a little larger, but the only fan is the CPU fan and that generally stays slow and quiet. It's time to update it with a new motherboard and put a 5700G to get 8 cores. The one shown in the pics is my software development machine and it's a dream for that.