Alright, so I got the mega to communicate, yay! Turns out my 10K from reset to 5V was cold soldered...
New issue:
It appears I get an error writing my extended fuse to 0xFD, see verbose below...
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x1e9704 (probably m1281)
avrdude: erasing chip
avrdude: reading input file "0x3f"
avrdude: writing lock (1 bytes):
Writing | ################################################## | 100% 0.01s
avrdude: 1 bytes of lock written
avrdude: verifying lock memory against 0x3f:
avrdude: load data lock data from input file 0x3f:
avrdude: input file 0x3f contains 1 bytes
avrdude: reading on-chip lock data:
Reading | ################################################## | 100% 0.01s
avrdude: verifying ...
avrdude: 1 bytes of lock verified
avrdude: reading input file "0xfd"
avrdude: writing efuse (1 bytes):
Writing | ***failed;
################################################## | 100% 0.11s
avrdude: 1 bytes of efuse written
avrdude: verifying efuse memory against 0xfd:
avrdude: load data efuse data from input file 0xfd:
avrdude: input file 0xfd contains 1 bytes
avrdude: reading on-chip efuse data:
Reading | ################################################## | 100% 0.01s
avrdude: verifying ...
avrdude: WARNING: invalid value for unused bits in fuse "efuse", should be set to 1 according to datasheet
This behaviour is deprecated and will result in an error in future version
You probably want to use 0xf5 instead of 0xfd (double check with your datasheet first).
avrdude: 1 bytes of efuse verified
avrdude: reading input file "0xd6"
avrdude: writing hfuse (1 bytes):
D:\Programs\Arduino\hardware\tools\avr/bin/avrdude -CC:\Users\You're_Nosey\AppData\Local\Arduino15\packages\MegaCore\hardware\avr\2.0.0/avrdude.conf -v -patmega1281 -cstk500v1 -PCOM5 -b19200 -Uflash:w:C:\Users\You're_Nosey\AppData\Local\Arduino15\packages\MegaCore\hardware\avr\2.0.0/bootloaders/optiboot_flash/atmega1281/optiboot_flash_atmega1281_115200_16000000L.hex:i -Ulock:w:0x0f:m
Writing | ################################################## | 100% 0.02s
avrdude: 1 bytes of hfuse written
avrdude: verifying hfuse memory against 0xd6:
avrdude: load data hfuse data from input file 0xd6:
avrdude: input file 0xd6 contains 1 bytes
avrdude: reading on-chip hfuse data:
Reading | ################################################## | 100% 0.01s
avrdude: verifying ...
avrdude: 1 bytes of hfuse verified
avrdude: reading input file "0xf7"
avrdude: writing lfuse (1 bytes):
Writing | ################################################## | 100% 0.02s
avrdude: 1 bytes of lfuse written
avrdude: verifying lfuse memory against 0xf7:
avrdude: load data lfuse data from input file 0xf7:
avrdude: input file 0xf7 contains 1 bytes
avrdude: reading on-chip lfuse data:
Reading | ################################################## | 100% 0.01s
avrdude: verifying ...
avrdude: 1 bytes of lfuse verified
avrdude done. Thank you.
And now it's not talking to me anymore:
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000 (retrying)
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000 (retrying)
Error while burning bootloader.
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000
avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
this check.
Did I just brick my board...?
Also, if someone else is trying to do this in the future, the boards file I included in the first post wasn't working, so I just used MegaCore's ATMEGA1281.