So I tried changing the Max FG Distance setting to keep my kicker from trying, and failing, at long field goals. But then this happened:

https://afl.myfootballnow.com/watch/3166#552713

Clearly I'm doing this wrong. How do I really set it up?

If it was me and since his career long (57) was at the start of the career plus he displays a -2 future overall career trajectory I'd push the sliders (under the Misc. tab) to 56 max at the end of halves/game and 54-55 regular. Something along those guidelines.

Once you set it up, it SHOULD lock in, therefore, not sure why it deviated from your setting!