This seems like it might be a good place to ask: does anyone know of a low-cost, readily-available SBC box with built-in dual Ethernet interfaces?
I've been very interested in some of Radxa's boards in the ~$30-70 range, like the E52C [0] and the E20C [1], but they don't have many distributors and seem to have stocking issues [2].
If you're okay with used, I got an "industrial" mini PC with loads of connectivity for maybe $125? https://support.onlogic.com/product-documentation/industrial... And there are plenty of configurations at lower prices, as well as many other manufacturers in this space.