Fusion Drive constantly spinning up and down
There are 2 ways to look what is it doing in sleep.

One is open the Console in utility, then look at the sleep hours log messages.

If you need help with that publish a copy of that segment here in your question.

The second way is using the Terminal in the utility.

In Terminal window type pmset -g

That will show you a list of sleep mode settings and who is preventing the sleep.