ionice equivalent for MacOS?

On macOS you'll find a similar program named taskpolicy that comes with the system. It allows you to both start a new program with a specific I/O scheduling policy, as well as change the I/O scheduling policy for an already running process. You will want to run it with the -d parameter, allowing you to set the I/O policy as either important (highest priority), standard, utility or throttle (lowest priority). There's also a level named passive that basically ignores priorities all together.

A simpler, open-source alternative also exists here:

https://github.com/DrHyde/ionice-MacOS

It allows you to set the I/O scheduling priority for a new command you start, but you cannot change it for an already running process or query an already running process for its priority.