You didn't mention what type of disk storage you are using, or if that matters. The number of cores in your database could also matter.
Does the max_parallel_workers setting have any influence on how effective_io_concurrency works?
Based on your data, one should set effective_io_concurrency at the highest possible setting with no ill effects with the possible exception that your disk will get busier. Somehow I suspect that as you scale the number of concurrent disk i/o tasks, other things may start to suffer. For example does CPU wait time start to increase as more and more threads are consumed waiting for i/o instead of doing other processing? Do you run into lock contention on the i/o subsystem? (Back in the day, lock contention for /dev/tcp was a major bottleneck for scaling busy webservers vertically. I have no idea if modern linux kernels could run into the same issue waiting for locks for /dev/sd0. Surely if anything was going to push that issue, it would be setting effective_io_concurrency really high and then demanding a lot of concurrent disk accesses.)
