Fresh install of macOS takes up too much space
I found this answer which says to download OmniDiskSweeper

It seems like my previous Downloads and Applications are still there but not showing under my File System. Somehow they are still on the disk, in a "hidden" place between both partitions.

I will try a new fresh install and I'll edit this answer later.

I managed to remove ~300 GB of the total space used by Previous Content using OmniDiskSweeper, but some items are "protected" against deletion, that's why a second fresh install might help.



After fresh install following these steps:


Shut down Mac
Turn it on again while pressing ⌘ R
Selected Disk Utility
Click on Erase
Closed Disk Utility
Selected Reinstall macOS
Waited for it to install
Success!


The picture below shows that, I've already installed some apps when I took this SS.