This is on my list of projects I want to try. Can you recommend a hardware platform to start from?

Edit: hardware for building a router that is.

My personal suggestion would be anything you have lying around your place that has supported graphics. Graphics are always the big end-user pain point for any OS.

Oh I meant for building a router

Honestly, any off the shelf or eBay x86 mini-ITX board is fairly well supported. Probably stay away from Atom CPUs (Celeron or i3 is fine) and any exotic hardware, use whatever ITX case you want and a PicoPSU, stuff it with a 2-4 port NIC and you're good to go.

Protectli [0] has a bunch of systems that should meet any number of price ranges and network needs. I've not personally tried OpenBSD on them, but I see nothing that should cause any problems. They're also small and fanless systems, which I really appreciate. Also usable as general purpose machine, so not limited to simple networking.

[0] https://protectli.com/product-comparison/

A router doesn't need a ton of power. My router is still a single core. There hasn't been a need to update it because my internet connection is only 50 megabit. If you have a lot of users and heavy traffic, then you will need more, but if it's just you and the WAN, you really don't need much at all.

I’m interested in the same thing. Also, what resources OP used for learning.

The thing about the BSDs, and especially OpenBSD, is that their MAN pages are phenomenally good. Documentation on their respective websites covers most of the rest of what you need. If you want to really nerd out, I highly recommend all of the No Starch Press BSD books by Michael W Lucas (the Absolute BSD books are a good place to start), but they are entirely supplemental.

“OpenBSD’s man pages are good” is a meme for a reason:

https://man.openbsd.org/pf.conf.5

That + https://www.openbsd.org/faq/pf/example1.html were more than enough to get me going.

As the other poster said, the man pages are great, but OpenBSD's homegrown daemons are also all very similar in style. A daemon ${service}d and a control interface ${service}ctl (ie. ntp/ntpctl ripd/ripctl ospfd/ospfctl relayd/relayctl pf/pfctl etc). The control interfaces are all designed to work similarly to each other, so they are very intuitive once you learn one. It's also worth noting that most services have separate manual pages for the daemon, the control interface, and the configuration file. I've also noted that many tools are very similar to Cisco IOS equivalents.

Unfortunately, the APU series from PCEngines was recently EOL'd, but they might still have some stuff available in the shop: https://www.pcengines.ch/

Throwing a NIC in an old box works well, but any board with 2 or more network connections will suffice.

As mentioned the apu2s were great but EOL now. They also didn’t push line rate when I upgraded to gigabit Ethernet.

I just went to a local computer shop and picked up a HP desktop which was likely off a business lease, then tossed another NIC in. It works a charm and routes gigabit just fine.

Odroid H3 is pretty good. Bought one myself as my (yet to be fully operational) home router.

It's small, quiet and has 2 ethernet for WAN and LAN and you can plug in an official USB Wifi dongle addon and it's good to go as a router.

You need to pick a few addons from the bare machine, like memory (I went with 8GB to run many containers), ssd (I went with m2 instead of emmc) and a case.

It's x86, so you'd have maximum compatibility for architectural differences.

https://www.hardkernel.com/shop/odroid-h3/

That sounds similar to my "origin story", I managed to get an older PC from a gamer friend who had replaced it and had been too lazy to get rid of it. I installed NetBSD on it after I failed to get my ISDN card to work on FreeBSD and set it up as a dial-on-demand router. And I also tried out a lot of other stuff related to system administration and networking - Apache, Squid, BIND. A few years later, I inherited an old SparcStation 20 and set up diskless boot to run NetBSD on that as well. Fun times, I can highly recommend something like this to anyone new to IP networking and Unix administration.