centos – Leapp always fails upgrade: more space needed

I’m trying to upgrade a SL/Centos 7.9 to AlmaLinux with Leap. I followed the instructions here but it always fails with

At least 3178MB more space needed on the / filesystem.

I’m increased the disk size by 5Gb and then another 5G but the message is identical. I’ve also set export LEAPP_OVL_SIZE=3000 according to some info, and then some more. No change.

$ df -h 
Filesystem           Size  Used Avail Use% Mounted on
devtmpfs             809M     0  809M   0% /dev
tmpfs                825M     0  825M   0% /dev/shm
tmpfs                825M  9,1M  816M   2% /run
tmpfs                825M     0  825M   0% /sys/fs/cgroup
/dev/mapper/sl-root   24G   15G  9,4G  61% /
tmpfs                825M   16K  825M   1% /tmp
/dev/sda1            497M  138M  360M  28% /boot
tmpfs                165M   48K  165M   1% /run/user/1000

Any idea what I can try next ?