@smeg the correct steps were:
1 - preload u-boot with command ./run_bootloader.sh -s T30 -t ./bct/grouper.bct -b …/re-crypt/u-boot-dtb-tegra.bin (note this is the unencrypted bootloader) AND simultaneously press the volume down button - it will enter the u-boot menu
2 - run the re-crypt script with additional parameter --split (to create two files)
3 - from the tablet, select fastboot mode from the u-boot menu, then follow the guide (fastboot flash 0.1 bct.img and fastboot flash 0.2 ebt.img)


@smeg to unbrick you can follow this guide https://github.com/tofurky/tegra30_debrick?tab=readme-ov-file#Nexus-7-2012-WiFi-Debrick
I think it’s sufficient to enter APX mode (follow point 3 on the above link) and then do the commands I posted