Pleasant thing about routers that is is so simple to build one after learning basics of networking and pretty much any OS or distro can act as one. There are obvious choices like OPN\PFSENSE, OpenWRT, DD-WRT, FreshTomato, but literally any PC with a single Ethernet port can act as one. My favorite setup was a laptop running Ubuntu and the whole router setup was in a single netplan file + dnsmasq for DHCP.

Edit: And ofc best cheap device imo is OrangePI R1 LTS and a whatever usb wifi dongle. Came in clutch a few times, such a nice little device.