It's worth learning nftables, it's actually pretty cool.

In most cases these days, "iptables" is just a compatibility layer for nftables, translating iptables commands and syntax to nftables rules.

The default fw in artix is nftables. It's just a wrapper for iptables, but you're best to make your changes in nftables, not iptables, if for no other reason than to not potentially break a future upgrade.

This is just the older init way to load sysctl and other modules.

When you move to or remain in an rc.d or init service mgmt system, you can't depend on systemd making your unit files and setting them up to load.

«I load ip_tables with modprobe, only to lose it after a reboot»

Write the module name into:
/etc/modules-load.d/.conf — on most modern systemd distributions (Ubuntu, Arch, Fedora, etc.)
or /etc/modules on Debian-style systems.

OpenRC's modules-load service reads configuration files from the following locations, in order of precedence
/usr/lib/modules-load.d/
/run/modules-load.d/
/etc/modules-load.d/

echo "ip_tables" >/etc/modules-load.d/ip_tables.conf