>/dev/mmsbl1 ,, I hope this is a typo
You can't mount whole card, only partitions,,,, /dev/mmcblk1p1
There is a systemd service (don't know name), or try startx or init 5
I just did a google search for "manjaro pacman history" and got "cat /var/log/pacman.log" So, if you ca get to a root console you might be able to find out what you removed. You can then try to reinstall with "pacman -Si <package>". I am a debian/ubuntu user, (but have also found that Manjaro seems to be the best distro on the PBPro) so check the commands I mention by looking at the man pages for pacman first.
If you can flash a SD card, on another computer, try the 20.04 SD card version of Manjaro. If it boots, you can use it to help.