but without nix it’s a pita to maintain through restores/rebuilds.

No it isn't. You can even define those routing polices in your systemd network unit alongside the network interface config and it will manage it all for you.

If you aren't comfortable with systemd, you can also use simple "ip" and "route" commands to accomplish that, add everything to a startup script and done.

major benefit to using a contained VPN or gluetun is that you can be selective on what apps use the VPN.

Systemd can do that for you as well, you can tell that a certain service only has access to the wg network interface while others can use eth0 or wtv.

More classic ip/route can also be used for that, you can create a routing table for programs that you want to force to be on the VPN and other for the ones you want to use your LAN directly. Set those to bind to the respective interface and the routing tables will take place and send the traffic to the right place.

You're using docker or similar, to make things simpler you can also create a network bridge for containers that you want to restrict to the VPN and another for everything else. Then you set the container to use one or the other bridge.

There are multiple ways to get this done, throwing more containers, like gluetun and dragging xyz dependencies and opinionated configurations from somewhere isn't the only one, nor the most performant for sure. Linux is designed to handle this cases.