So what is a good router nowadays? I'm about to buy one and I'm not looking towards this particular rabbit hole.
TLDR: Any standard PC with two or more Ethernet interfaces and OpenBSD with PF.
I like the small Qotom Q305p 3205u computers for this, but any small device will do and if you don't care about the router being small and quiet, any form factor PC will work... just need two or more NICs to route packets from one network to the other.
I really like my Cudy Wifi routers. They can run upstream openwrt (double check your version). They run some modified openwrt.
They're both cheap and quite good. They're what GL.inet used to be when i last purchased routers before this.