Have you worked with pfsense before? If not I’d buy a cheap HP T620 or T630 and set up router on a stick, or buy a more expensive T620 Plus with a quad port NIC and give it a test.
You can do 2 NIC connections on the T620 and T630, but you need to get one with the fiber optic option. If you are lucky, you might find one for around $40usd. You can also try a USB NIC, but I’d label this “for testing only”, tried it, lived with it for a while, gave up and bought what I needed to do the job correctly. The T620 Plus is normally going for around $160usd including the 4 port Intel NIC card and normally including shipping. I’m not pushing my T620 Plus very hard, so no sure if it will handle everything you want, but it gets you going. Serve The Home has some articles on their website about buying Tiny/Micro systems ( Tiny, mini, micro series) and setting them up for pfsense or Plex, might be worth a look. You can get some Intel i5 processor machines for a few hundred usd that would probably handle what you want to do.
That said, the SG-5100 should do it, and it seems the prices have come down since the last time I looked at their products. I remember the SG-7100 being more money than the current price, I was pushing to get one of those a few years ago, but no budget and cannibalized a different server for the job after working out an alternate plan.