What is the maximum recommended routing distance between MAC and PHY?
MII and RMII have round-trip time restrictions, since the TX data path on MII is destination synchronous, and the RX data path on RMII is destination synchronous.
Both RMII and MII hit a hard timing limit at a few hundred millimeters trace length.

RGMII on the other hand is fully source synchronous, which means it has no hard timing limit on the trace lengths.
It simply becomes an exercise in signal integrity and length matching.
With for example length and impedance matched coaxial cables and terminations you should be able to go quite far.

That said, SGMII or 1000BASE-X is better suited than RGMII for board-to-board ethernet backplane situations like this.