Half off-topic, sorry: if you have some spare time on the weekend, you might want to take a look at nftables. AFAIK iptables is also just using nftables under the hood, so you are basically using a deprecated technology.

nftables is so much nicer to work with. In the end I have my custom rules (which are much saner to define than in iptables) in /etc/nftables.conf, then I have a very simple systemd unit:

[Unit]
Description=Restore nftables firewall rules
Before=network-pre.target

[Service]
Type=oneshot
ExecStart=/usr/sbin/nft -f /etc/nftables.conf
ExecStop=/usr/sbin/nft flush table inet filter
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

and finally if I push updates via ansible I simply replace the file and run nft -f /etc/nftables.conf (via ansible; on-change event).

Edit: oh and as an example how the actual rules file looks like:

#!/usr/bin/nft -f

add table inet filter
flush table inet filter

table inet filter {
  chain input {
    type filter hook input priority 0;

    # allow established/related connections
    ct state {established, related} accept

    # early drop of invalid connections
    ct state invalid drop

    # allow from loopback
    iifname lo accept

    # allow icmp
    ip protocol icmp accept
    ip6 nexthdr icmpv6 accept

    # core services
    tcp dport {80, 443} accept comment "allow http(s)"
    udp dport 443 accept comment "allow http3"

    # everything else
    reject with icmpx type port-unreachable
  }

}

and with that I have my ipv4+6 firewall that allows pings and http

I have used both, can tell you that a template file of /etc/iptables/rules.v4 with iptables-persistent is the easiest way.

if you go the full IaC route and have vars for the rules, remember to get iptables to save its state after you have applied your rules

I second the use of nftables instead. Optimally with a pre-made role like this one: https://galaxy.ansible.com/ui/standalone/roles/ipr-cnrs/nftables/documentation/

If I understand you want iptables to be persistent across reboots? Would the following be useful?:

apt-get update -y && apt-get install iptables-persistent -y
service netfilter-persistent save

I have no clue about ansible as I have not explored that region of selfhosting yet. It's on the list tho.

Generally, you set up a rule + command playbook, where the command invokes the iptables-save command.

You want something outside of IPtables like Firewalld. Ansible should only run to make changes to a existing system.

For your information, iptables should not be used anymore. It has been deprecated. Nowadays you should use nftables, it's successor made by the same company.