firewalld is a front-end for nftables and iptables.
You are probably using nftables with firewalld. In this case, I would recommend replacing iptables with iptables-nft. That way, anything that depends on iptables will still work but it will be backed by nftables
Are you talking about the iptables.service? That doesn’t actually enable or disable the functionality. It just loads the rules from /etc/iptables/iptables.rules. That should be disabled by default. I don’t think that needs to be enabled unless you want to manually manage iptables rules. I believe it can be disabled even when using ufw