Hmmm... I tried to nspawn openwrt in the past on x86 and had no luck... there was a thread about it: Using Systemd to run Openwrt on Raspberry Pi 4…welcome to join
are you sure OpenWrt is starting up fully? If so, what did you have to do to make it work? (feel free if you like to add to info in that other thread, it's not closed yet).
also, the nftables vs iptables stuff is definitely an issue when running in a container. It wouldn't be as much of an issue in a fully virtual situation, but that wouldn't perform as well either. One of the big issues is that nftables and iptables have totally different NAT modules.
Honestly, these days I run Raspbian as my router. I set the network up with systemd-networkd files in /etc/systemd/network (update to latest systemd) and do the firewall in nftables. nftables is FAR preferable to iptables. I put the log on a separate flash partition with jffs2
The only thing you don't get is luci, but I run netdata on the device to get a quick snapshot of how well its working, and do all my configuration in ssh.
I don't miss the lack of full set of modules, specialized tiny binaries, and etc.