If you do want the USG back, most of their failure modes (flash storage, PSU) are fairly easy to resurrect
I've also looked at installing OpenWRT on my USG, but only the snapshot version is available and the process is fairly involved[0] so I haven't really explored this.
At this point though, I'm not likely to go back to Unifi for routing. I needed to use the config.gateway.json (I think) workaround to add some DNS config[1] to my USG, whereas with OpenWrt I can do this entirely in the UI.
[0] https://openwrt.org/toh/ubiquiti/unifi_security_gateway_3p
[1] Just wildcard resolution so that *.internal.domain points to a reverse proxy server, nothing too extraordinary.