Yep, some always-on device with local storage is simply a requirement. As pointed out already it is quite easy to have an un-updated peer come online when all updated peers are off.

So that leaves you with the need for at least a small NAS device always left powered on. This can range from a USB flash drive pluggd into a router supporting NAS to a dedicated device to do the same thing. These range from small and cheap to larger and more reliable:

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However if this is about serious home automation you should consider that SOHO routers with onboard switches and WiFi access points have "blind spots." Many of them disable the Ethernet switch and WiFi radio and can't serve DHCP while being updated or rebooted, and they can freeze in that state. You may find you need to "reboot" your router for lockups even more often than you do the cheapest of NAS devices.

The normal solution is a "hard" Ethernet network with a reliable dedicated switch and preassigned fixed IP addresses for critical devices. This can then be connected upstream to the multifunction "router" though you may have to configure the DHCP range to be smaller than the entire subnet to leave unmanaged space for the fixed IP addresses.