Requirements do vary.

And I've got no regrets about using a Pi 4 a router. It's sitting on the shelf next to me, and has been trouble-free for over four years now.

How much stuff is hanging off usb on it instead of being in a proper case?

You may not care but I do.

I suppose a Pi 4 is fast enough to route 1 Gbps even through a usb network card?

[I do have one "sitting on a shelf" as a home Minecraft server btw. I hate the usb ssd hanging off it.]

There are some nice cases for RPi 4 that have a semi-integrated USB-SATA adapter in the box. The DeskPi[1] case, Argon and the NESPi-style case in particular. DeskPi is pretty neat, if costly, and the NES-like cases have easy 2.5" sata swap. YMMV.

I upgraded my home LAN to 10gb/2.5gb early last year, and the RPi can't really keep up with it anymore. I'm using an N305 based mini-pc with 4 ethernet ports for my router now... exceeds my needs and runs great. My home server is an AMD 5900HX based mini-pc with also works very well (ProxMox, Docker, etc).

The only USB thing it has is a NIC. But given the speed of my ISP I could get around that by using VLANs, I guess.

I mean: There's not much to it. LAN port, WAN port, MicroSD storage, and a (completely optional and of dubious merit) hardware RTC that sits on the GPIO header. I'm using the bog-standard Pi Foundation plastic case.

It runs OpenWRT, which itself is very light-weight -- it's more-or-less just enough Linux to set up routing functions using a GUI, and it's meant to be able to fit in the tiny little bit of flash that consumer router hardware comes with.

IIRC, the status at the time when I put it together was that it could route packets and perform traffic shaping at ~1Gbps even with a USB NIC. It certainly did just fine when I had a 400Mbps pipe.