I found the issue. As @tkaiser pointed out, md raids aren't the best. My raid configuration was the bottleneck resulting in high I/O whenever I turned my TV on or my smart home pushed multiple sensor data.
I switched to an additional SSD for all the docker stuff, so this is separated from my data. Data is now on one 6 TB disk, formated with EXT4. For the second disk I was thinking of using the rsync method to have the data duplicated on both disks.
Any hints / ideas on how to implement the rsync method? Is this a good approach?
@m4dm4x Hope you've fixed your issues, too.