MariaDB I/O capacity with innodb_io_capacity parameter
It looks to me that you should increase that parameter - what I would advise is that you set up database benchmarking  and vary the parameter in steps of 1000 until you find a sweet spot. There are any number of MySQL benchmarks - sysbench for example. You could try mysqlslap - it's "quick and dirty" but should be good for a first approximation - 1, 2, 3, 4, 5 and check out the documentation. I've been messing with this recently so I have the links to hand.