If computers are in same network, even with different ip addresses, they still can see all broadcast and multicast traffic. This means for example dhcp.

If you fully trust your computers, and are sure that no external party can access any of them, you should be fine. But if anyone can fain access any computer, it is trivial to gain access to all networks.

If you need best security, multiple switches and multiple nics are unfortunately only really secure solution.

Luks FDE, and install dropbear-initramfs, configure ssh authorized_keys and rebuild initramfs. Then you can access initramfs via ssh to type luks password.