This can be a language problem. All traffic on the LAN is just on the LAN, peer to peer. You can not let it go “through a box”. And for traffic going out of the LAN, see what @arvidjaar says.
It is also not clear what the present situation is. Are you using no DHCP at all on the LAN and are all boxes configured with fixed IP/Netmask, default route and DNS servers? Or are you using a DHCP server on the LAN and want to remove that and switch the service to a one of the other boxes which runs openSUSE?
@gregrwm probably better to find a router than you can run openwrt on and play with that… else look at setting your router in bridge mode and serving up everything on a dedicated system that can serve up both ethernet and wifi access…
If your only experimenting then maybe a couple of virtual machines and test between them.
You did not describe your environment so I had no way to guess it.
no (depending on what you call “systemd” of course)
NetworkManager supports using DHCP for address configuration of internal systems in “connection sharing” mode which implements exactly your requirement “to channel all traffic on my lan thru a box”. Of course you may have some other additional requirements that are not fulfilled by it.
“Best” is in the eyes of beholder. But the obvious answer is - use DHCP server (package dhcp-server) if you need DHCP server.