I had a very similar problem as @Toralv@lemmy.world a few weeks ago. I repurposed a small, fanless x86 desktop computer as my new router. It has only one RJ45 port and due to its small size cannot be extended with a proper network card. As it has an unused USB3 port, I acquired a cheap Realtek-based USB3-to-RJ45 ‘adapter’ as the second network interface. It works without any further issues in Linux (Arch) and has no problems to handle Gbps traffic.

For the router configuration, I am using ‘nftables’ instead of ‘iptables’, as the former is supposed the successor of the latter. I only used the new nftables configuration, but there are wrappers available so that one can continue to use iptables syntax if desired.

For network configuration, I am using systemd’s networkd. Check systemd.network(5): Configuration option ‘IPMasquerade’ takes care of telling nftables/iptables to setup masquerading (rendering the iptables invocation @thebardingreen@lemmy.starlightkel.xyz exemplified unnecessary), options ‘IPv4Forwarding’ and ‘IPv6Forwarding’ renders manually changing ‘/proc/sys/net/ipv4/ip_forward’ unnecessary.

systemd’s networkd has a built-in DHCP server; check option ‘DHCPServer’ and section ‘DHCPServer’ for that (same man page as above). This way you can skip installing/configuring a separate DHCP server, but systemd’s DHCP server has some limitations, such as only supporting DHCPv4 and lack of proper command line tools. For example, to retrieve the list of current leases, you would have to make a dbus call to networkd, e.g. via busctl or dbus-send.

Bridges can also be configured with systemd’s networkd, making a separate bridge tool unnecessary. Rather straight-forward with three small configuration files, telling networkd that you want to have a bridge, its name (e.g. br0), its MAC address, which NICs will be part of the bridge, and the bridge’s configuration like a NIC itself (e.g. static IP address, that the networkd’s DHCP server shall listen here, …).