I have a few questions :
1) Is it mandatory to upgrade U-boot, on newer pbp ?
Yes, if you want assistance, I have no idea what manjaro uses, I do
know it's on the EMMC.
2) Can I know what version it has without serial port ? Missing
config file for fw_printenv.
The Fedora one? It's 2021.04 with some fixes. The Fedora one outputs
all the version information to the screen. No clue on manjaro one,
I've never used it.
3) Can I write U-boot from manjaro ?
No idea, I've never tested it.
4) Is the pcie support is not already enabled in manjaro ? Could you
just use it's kernel config like
https://github.com/bengtfredh/pinebook-pro-fedora-installer.
See the bug report linked in my blog post, in short "No, it's not that
straight forward"