With DIY you are usually left with at best ripping it all out and selling it without any “Smart Home” promises, you can I guess still market that it has structured cabling to enable smarts though?
Edit: The only "problematic" part would be the cameras. Not sure if Unifi cams can work with other platforms than Ubiquiti's. But in any case, the wiring is there (Ethernet) and the cameras can be replaced or a new owner could bring their own Unifi surveillance console (or we could sell it with it).
https://help.ui.com/hc/en-us/articles/221314008-UniFi-Video-...
I mostly ended up and recommend Axis for IP surveillance, it has nice MQTT integrations and recessed mount options (https://www.axis.com/products/axis-t94s01l-recessed-mount) but almost anything which can do RTSP will work with either a COTS NVR, or something like Frigate (https://frigate.video).
Another thing to read up on and use when shopping for IP surveillance is ONVIF and the various profiles offered!