Please contact your hardware manufacturer on how to reset the UEFI firmware (you don’t have a BIOS, you have UEFI firmware).
That’s normal: “Devices” shows the non-root devices but you have none any more: everything is allocated to Manjaro’s root FS right now.
Please read this:
Especially the section “How to install software”…
No one can answer that question, except you. However, if you do decide to reset the UEFI firmware according to your manufacturer’s instructions, you’re in for a reinstall anyway and I’d take:
- 1G for EFI,
- 64G for
/
, - RAM+SQRT(RAM) for
swap
- 800G for
/home
and leave the rest unallocated.