If you use a firewall to restrict systemd, your system breaks.
I don't understand why you would do that or what you would expect to happen, systemd itself is the tool that implements service-level firewalling.
Last time I checked, iptables and nftables filtering was done by the kernel, and userspace tools other than systemd existed to configure them.
I'm still not sure I understand, what is it you're trying to do exactly? Those other tools should continue to work, systemd will only conflict if you explicitly enable its eBPF stuff.
You firewall against network connections, not against PIDs. What is PID 1 connecting to/accepting connections from that blocking them breaks the system?