My USG-3P died a few months ago, so I dusted off my old Asus router and made an unplanned switch to OpenWrt. This was meant to be a temporary solution until I could get a new UniFi device, but after setting it up and getting it working the way I want I'm more than happy to make this temporary solution into a permanent one.
My Asus router, when using OpenWrt, can't keep up with my internet connection, so I'm putting (admittedly minimal) effort into finding a suitable upgrade with the intent to keep the Asus as a backup.
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.
Just pick up the nanopi r4s or r6s. Fairly small, but with nice specs and support in openwrt. The r6s isn't supported by openwrt 23.05 but you can get the friendly elec fork or get a community rebuild of openwrt with a later kernel that supports it. You'll get a router that can work at line speed (1Gb or 2.5Gb) even if you use features like SQM or wireguard.
Try a raspberry pi 4 with an extra gigabit USB adapter. Just make sure you use the custom firmware builder to include the USB networking packages. Easily fast enough for gigabit speed advanced QoS handling. Just don’t use it for its Wi-Fi.
My USG-3P died a few months ago, so I dusted off my old Asus router and made an unplanned switch to OpenWrt. This was meant to be a temporary solution until I could get a new UniFi device, but after setting it up and getting it working the way I want I'm more than happy to make this temporary solution into a permanent one.