For anyone who prefers a more minimalist approach to managing ip[6]tables rules, I have created a simple init script, which can be found at https://github.com/ipilcher/openwrt-iptables.
Note that the provided configuration files enable only ssh. (IP forwarding and NAT are both disabled.)
Simplicity - I use my OpenWRT device as a wireless access point only. It's basically a layer-2 device; layer-3 is used only for administrative access. Thus, I don't need the majority of the functionality that the standard tools provide.
Familiarity - I've been using iptables and ip6tables "directly" for almost 20 years now on Red Hat/CentOS/Fedora distros. (I haven't migrated any of my systems to firewalld.) Since I only need a very simple setup (see point #1), I prefer to use the same tooling that I use on all of my other systems.