Great investigation, thanks for putting in the time and effort.
But for a server with relatively low user traffic, 347 GB/day of writes feels disproportionate, especially compared to the Cloudron demo server at ~80 GB/day.
I actually fixed some graph bugs yesterday here and here. Is that number 347GB coming from the Cloudron graph? If so, that value is actually showing the value since the server last rebooted! It has nothing to do with the window range selected. I also noticed that if you select ranges, you will see the value decrease. This was the bug I fixed.
@imc67 also, have you compared the i/o rate against your VPS provider graphs also? I wouldn't rule out a bug in cloudron graphs (we rewrote the metric system, so maybe there are bugs).