Ask HN: What filesystem would be best for this specific scenario?

I need some advice about choosing the best filesystem for a specific situation.

-there are 26 directories (A-Z, plus another one);

-inside each directory, there are thousands of files (will slowly reach tens of thousands in the next decade);

-the files are small (between 0.1MB and 5MB)

-the files reside on a slow medium (USB memory and/or hard disk that is 90% in standby i.e. needs to spin up before any I/O operations);

-the files will be accessed locally AND via network share;

-most of the I/O operations will be READs; WRITEs will be very few (estimate: <5% of the I/O operations); thus, READ performance is much more important in this scenario.

As the OS doing the I/O operations is Linux, any filesystem supported by it is being considered.

Please weigh in.