If the fragmentation is below 10% it isn't worth defragmenting. However, if it grows 5% per week, that would seem to indicate something other than MC is writing and modifying files. I guess that you use the Dell for other activities?
Anyway, Tom's Hardware says this about defragging:
On your computer with Windows 10, defragmenting only the system drive (C: ) is sufficient. However, if you experience long load times while accessing the files stored in other partitions of the hard disk, you can defragment them as well
Microsoft says this:
Once Windows is finished analyzing the disk, you can check the percentage of fragmentation on the disk in the Last Run column. If the number is above 10%, you should defragment the disk.
So, opinions, everyone has one.
BTW, I would have though that the Microsoft defrag process, even though scheduled, would only happen when the PC is idle. But I can't find a reference for that. I also can't check the settings you mention, as Norton 360 has taken over control of defragmentation on my PC, so I can't access the settings. Norton "Optimisations" only run when the PC is idle, and if I had defrag turned on it would run with them.
So, is your PC idle at 8:30pm? Maybe it tries to start, and you see that, but then it stops when the PC is in use? I would guess you need to find a Microsoft reference.