One option I'll toss in also is using one of the "mini/micro" machines as a pfSense unit. There are many blog postings out there for how to do that but there can be gotchas. Like not enough ports or even the slots, or physical connectors to add them. Then there are the gotchas of using genuine parts. Most boils down to how much time do you want to tinker/spend vs
get an out-of-box working unit up/running?
With the pre-build units like ProtectLI or NetGate, you get a ready-to-go appliance, without a lot of those headaches to make it work. That's the KIS route that is sure to take the least time.
If you decide to tinker, many of the tiny-mini-micro-thins make fine pfSense units and these surface on eBay all the time - Dell, HP, Lenovo all sell units. Lenovo has an option to add a 4-port Intel pNIC to one of their Tiny models - but you may find the $ a bit higher than you want to spend - new. (ouch!) Buying used you have to know which models make good options b/c some may not even have the parts, slots or ability to add something like 4 port Intel card to and never assume. These manufacturers do not put the ability to add or parts on the motherboards to add something like a pci-e slot, if they never intend the unit to have/hold any sort of pcie card! Every .01 counts! There may also be special brackets to hold cards in place or you do a "make it work with a dremel...."
Finding the right unit again is the issue b/c many times, they only have a single port which is often broadcom-based and pfSense works and support is much better with Intel pNICS. My biggest hang up is understanding the setups enough to make sure I don't leave the doors wide open by mistake and invite a world of trouble.
Search for pfSense guides on minis there are several. Just a few I've hit over the years as I've looked at this option as the main front-door router.
1.
https://www.servethehome.com/netgate-sg-2100-pfsense-router-and-firewall-review/
2.
https://www.servethehome.com/guide-tinyminimicro-pfsense-firewall/
3.
https://www.thegeekpub.com/14863/the-best-pfsense-box/
4.
https://www.servethehome.com/lenovo-thinkcentre-m720q-tinyminimicro-feature/
5. Fake 4 port Intels ->
https://forums.servethehome.com/index.php?threads/comparison-intel-i350-t4-genuine-vs-fake.6917/
6. Doing it with 1 port -->
https://www.joe0.com/2019/11/16/con...irtual-lan-configuration-on-a-managed-switch/
7. HP example ->
https://boratory.net/hp-t730-thin-client/
8. ...
Example PITA of finding the right model and then adding the right parts ->
https://forums.lenovo.com/t5/ThinkC...0-720-910-920q-TINY-series/m-p/4612559?page=1
BTW, me and a buddy have toyed with doing this for about 18 months now and so far,
we've decided for now to remain with our tried, true and trusty Asus AC86U units + Merlin + AMTM +.... . For me, I just do no have the cycles to take on another project or have the family
screaming about the internet being weird or down. Good luck!