PostmarketOS rant / plea for help

Has anyone actually successfully installed PostmarketOS on an old device recently? I've had a long struggle through trying to prepare a Nexus 7 (2012) and the result seems to be a dead device before I even got to actually installing PostmarketOS.

The rough steps I followed are listed here:

• Create backups
• Get SBK
• Build and prepare U-Boot
  • Actually flashing U-Boot seems to be where things went wrong
    • Running ./run_bootloader.sh -s T30 -t ./bct/grouper.bct -b ../re-crypt/repart-block.bin or ./run_bootloader.sh -s T30 -t ./bct/grouper.bct -b ../generated-wheelie-blobs/AndroidRoot/blob.bin from fusee-tools hung on waiting for bootloader to initialize
    • Running ./run_bootloader.sh -s T30 -t ./bct/grouper.bct -b ../u-boot/u-boot-dtb-tegra.bin failed like this
    • skipping that step and running ./utils/nvflash_v1.13.87205 --resume --rawdevicewrite 0 1024 ../re-crypt/repart-block.bin hung on [resume mode]
    • Consulting a different version of the docs and running ./wheelie --blob ./generated-wheelie-blobs/AndroidRoot/blob.bin seemed to work so I ran ./nvflash --resume --rawdevicewrite 0 1024 ./re-crypt/repart-block.bin which also seemed to work
    • I then powered off as instructed and the device has been completely unresponsive since

I've tried connecting to a charger, disconnecting and reconnecting the battery, and every combination of holding down buttons but it appears to be completely dead. Any suggestions as to what I did wrong or anything I might be able to do now? Obviously it's not the end of the world to have lost a 13 year old tablet that was just gathering dust, but at the moment I'm not feeling positive about ever trying this again on another device!