Luke- I was able to compile successfully using 1.8.13 IDE and AVR board # 1.6.20.
The memory usage was as follows:
Sketch uses 24238 bytes (78%) of program storage space. Maximum is 30720 bytes.
Global variables use 1450 bytes (70%) of dynamic memory, leaving 598 bytes for local variables. Maximum is 2048 bytes.
Well done and thank you again for looking into, and solving, this.