Not enough space to upgrade Mac OS via bootable USB

I'm trying to upgrade an oldish MacBook AIR from (I _think_) High Sierra to Big Sur. I've made a bootable USB and it's working fine: - Power ON with holding Option/Alt - Select USB boot - Goto Menu - Install BigSur It lists 2x HD's - Macintosh HD - Macintosh HD - Data when I try and install this OS onto either, it says I need 9GB free space. I have no idea how I can free any space on this (either?) partitions because I can't log into the machine. If I let it try and boot up _normally_ it hangs in some "install?" screen with "29 minutes remaining" and hangs there for ever (i've had it on for over 24 hours). So -> I'm hoping there might be some place I can use Terminal to manually rm some files, like the user's recycle bin or downloads or something? I can't seem to see _any_ real files when I use Terminal from the INSTALL Menu :( It's like .. everything is locked down/hidden? I did fine ONE link to help me which suggested I remove the VM (virtual mem) disk, so I did that which released 2GB and now it's down to 7GB needed. Can anyone help, please? Maybe I need _another_ boot disk which can access the HD with some nice UI (like some ASCII GUI like the old days) ? EDIT: - Added pics - Apologies for the blurry 2nd pic EDIT2: I need to keep the data on the disks so I can grab it / back them up. So i cannot Erase the partition(s) with Disk Util.