As far as I know, the Shelly Pro 3EM has a memory for 60 days (with minute-by-minute measurements). So reading it out once a year will not be enough,
but about every 2 months. As I switch the router off for 6 hours at night, I can confirm that the storage basically works, as all the data is available the next day after switching on the router.

The data is stored in the device. I mean 12 months.
It doesn't really matter how you establish the connection.
I recommend doing everything via WLAN and the cloud.
If there's no Wi-Fi and it's important, then you can book Starlink and really always be connected in Europe.
I've been doing this for years.

Hello, my Shelly 3EM pro is permanently connected to the LAN so I don't have the opportunity to test but according to the documentation it would seem that the log capacity is 60 days.

Sorry about that but I can't say that with me it always hangs in the wired network.
You can understand it up to one year [I have it since february 24].