I am one of those other lame-Windows guys which can't run Gparted from a WSL ( access to raw disk )...
Any advices on extending the partition just with the pre-mounted packages of the "images" found  with your link ?
The 3x Debian builds do not even have 'parted' installed and executing apt-get update BEFORE extending the partiotion extension doesn't leave enough room to install parted ,,,
I tried  a few times using fdisk directly but the result is always the same -> bricked
--> mandatory BelenaEtcher  sdcard
fdisk -l  ( results )
Device                     Start          End     Sectors  Size Type
/dev/mmcblk0p1    34              200          167 83.5K Linux filesystem
/dev/mmcblk0p2    201            367          167 83.5K Linux filesystem
/dev/mmcblk0p3    368          3385        3018  1.5M Linux filesystem
/dev/mmcblk0p4    3386        7481        4096    2M Linux filesystem
/dev/mmcblk0p5    7482   2104633 2097152    1G Linux filesystem
root@CoreMP135:~#
-deleted last partition ( p5 )
- created new parttion
 start @ 7482
 End   @ default much large value presented
 type  @ default
- write  ( save )
reboot
BRICKED !
any advice ?
So close ( but still so far .... ) to getting to start using my CoreMp135 !  :-)