We were able to make some progress with @Thatoo on the chat who encountered the same issue
I crafted a small hack that removes the dependency to build-essential from the $app-ynh-deps :
sudo cp /var/lib/dpkg/status /root/dpkg_status.bkp
sudo perl -i~ -0777 -pe 's/(Package: .*-ynh-deps\n(.+:.+\n)+Depends:.*)(build-essential, ?)(.*)/$1$4/g' /var/lib/dpkg/status
After which we still had to :
# (NB: READ CARREFULY what the next command says
# and make sure it doesnt remove half of the system ...)
sudo apt remove build-essential gcc-8 libgcc-8-dev
The command should display something like :
The following additional packages will be installed:
binutils binutils-arm-linux-gnueabihf binutils-common cpp cpp-10 gcc gcc-10 gcc-10-base libasan6
libatomic1 libbinutils libcc1-0 libctf-nobfd0 libctf0 libgcc-10-dev libgcc-s1 libgomp1 libisl23
libubsan1
Suggested packages:
binutils-doc cpp-doc gcc-10-locales gcc-multilib flex bison gcc-doc gcc-10-doc
Recommended packages:
libc6-dev
The following packages will be REMOVED:
gcc-8 libgcc-8-dev
The following NEW packages will be installed:
cpp-10 gcc-10 gcc-10-base libasan6 libctf-nobfd0 libctf0 libgcc-10-dev libgcc-s1 libisl23
The following packages will be upgraded:
binutils binutils-arm-linux-gnueabihf binutils-common cpp gcc libatomic1 libbinutils libcc1-0
libgomp1 libubsan1
NB : if this displays MUCH MORE packages than this, this is not good. Otherwise, you may safely proceed
After which, re-running the migration should be ok