I’ve done this in the past and had great results. The only downside is that running a regular PC drawing ~100W 24/7 can easily add up to $100/year depending on electricity costs and eventually an embedded device would pay itself off.
Can't say if it applies to your case, but as a firewall/router I use a "thin client" with a TransMeta processor, the actual model is Fujitsu Futro S, there are/were several sub-models, mine is an old S220, it runs Zeroshell (a Linux distro) with an added "normal" PCI network cards and it is like 15W:
Thin clients were fun, but they aren’t nearly fast enough for modern multi-gigabit Internet connections. They can route maybe up to a couple hundred Mbps.
I’m wondering whether a mini-PC can route even 1 Gbps at line rate. Lots of people are using mini-PCs as routers, but most of them have only 1×1Gbps Ethernet interface and no PCI slots. The Minisforum DMAF5 has 2×1Gbps Ethernet interfaces, but that’s an off-label use and I haven’t seen any benchmarks.
Of course, if you go all the way up to mini-ITX, then there are plenty of options for various performances of CPUs and network interfaces.
I can't say for sure what you can do with a miniPC, but I did 1 Gbps routing with all packets going through userland and back to the kernel (because of any over elaborate at&t router bypass) on a pentium g3258 (haswell, dual core, 3.2 Ghz) and it was fine. A transmeta box seems probably a bit old, but relatively few people have gigabit internet. If you wanted that box to do wifi too, that might be problematic; but I generally use wireless routers configured in access point mode to be access points. In access point mode, as long as the networking hardware and drivers are decent, you get fine performance; you don't need great NAT acceleration or properly managed memory for state tables on the wireless devices, since that's managed on the router/firewall/could be a home server too.
Benchmark sites like userbenchmark.com and cpu-monkey.com rate the 7-year-old 3.2 GHz Haswell as having roughly the same single-core performance as the 2-year-old 2.1 GHz (3.7 GHz single-core turbo) Zen+ CPU in the DMAF5. But that doesn’t cover the I/O performance, especially with Realtek Ethernet interfaces.
Most mini-PCs are using 2-GHz-or-lower Gemini Lake CPUs with much worse single-core performance, but much better video I/O performance. But that still doesn’t cover network I/O, and Realtek vs Intel. (And Intel i40 vs i211.)
Relatively few people have gigabit Internet, but it can happen suddenly. 3 years ago, I upgraded from 20 Mbps DSL to 1 Gbps symmetric fiber. Bye-bye to my old MIPS 24K router. But all these years later, my brother 1 mile away is still waiting.
