My hardware is wrd841n v11. I can test some basic functionality. But I'm testing my build now. So far been running > 24 hours without issues using it as mwan3 load balancer and wifi router
I have taken your ultra lite and added mwan3, sqm, AdBlock, upnp, libustream, uhttpd, luci app and removed opkg.
I didn't test your openvpn build but I have notice my build have free memory less than 2mb which using Luci will cause out of memory and the wifi and lan port will reset itself.
It's really hard/frustrated to configure using Luci due to this
Workaround is to disable AdBlock service.
After configured/finalized, re-enable AdBlock and disable uhttpd (Luci) and reboot router
If need to configure router, use SSH to enable uhttpd and disable AdBlock
Definitely not user friendly