ethernet wire + old laptop (desemble no lcd no keyboard hide it in the box etc in the wall) + flash directly dietpi on hdd/ssd put it back in the wall + ssh

ethernet wire + old laptop (desemble no lcd no keyboard hide it in the box etc in the wall) + flash directly dietpi on hdd/ssd put it back in the wall + ssh in to it and encrypt hdd or ssd + put firewall openvpn (for reaching over wan) = NextCloud is your sollution rpi might work too, but slow. But if you are paranoyed that someone can steal your *photos. From your home. Then use batteries from old laptop, take them out 3.7V 6 or 8 of them and use few for cloud backup if main power is off and for powerup rpi with cammera for face detection and movement sensors and main electricity inficator and usb 3G or 4G ethernet dongle if that thief will turn off electricity you can get notification via 4G network. If electricity is not turned off you still can get motion detection. Use rpi gpins to send electroshock to burn that hdd. Good luck experimenting.