since the IS31FL3733 is not recommended for new designs, why not redesign the LED PCBs with the IS31FL3741. gotta research that - would mean more LEDs (117 instead of 64).

According to my table, I would get 2 more rows around the cube with 106 LEDs. 

There's only one ADDR pin here, so you can have a maximum of 4 panel adresses, but definitely managable to address half a cube with the RP2040 that has two I2C channels.