Hi Gerson. The installer currently doesn’t make it super obvious how to do what you want, which is to preserve the existing /home contents, and create a new / to install to.
The installer enforces the creation of a new file system for / when on ext4 and XFS but it’s different on Btrfs. There’s a partial exception: you don’t need to reformat the Btrfs volume, but you do need a new subvolume for /.
Fortunately the installer will create a new subvolume for you if you add a new / mount point rather than reusing the existing one. You can optionally delete the current / in the installer or do it post-install, it really depends on how much space you have and how attached you are to any of the contents in the “old” /
There is a Fedora QA test case that describes how this should work. It’s not pretty because it’s not a user facing document (yet), it’s targeted at testers. But it should help you navigate the installer’s idiosyncrasy for your use case.
https://fedoraproject.org/wiki/QA:Testcase_partitioning_custom_btrfs_preserve_home
Your screenshot above is pretty much at the “How to test” step 6 portion of this test case. Make sure you check step 5, and ensure that the partitioning scheme has been set to the default: Btrfs. The manual partitioning UI enforces rules based on this partition scheme selection.
Let us know how that works for you, in particular if you get stuck at any step along the way.