I decided to try making a custom kernel (5.9.10). It compiles and installs without issue, but when trying to boot the splash ends with 
 mount-pts:fatal: crash!
My keyboard isn't responsive during this time, so I can't access tty12 and /run/66/log/0/current doesn't seem to exist after chrooting in from my recovery usb.
It's easily recoverable by just timeshifting back to this morning, so I'd just lose a couple of small updates. But I'd prefer to fix the issue.
Both grub and init verbosity is at the highest
This service try to make the /dev/pts as a mountpoint with the options: 
-o mode=0620,gid=5,nosuid,noexec
you can skip this mountpoint at boot time setting MNT_PTS=!no. To do so (as root) 
# 66-env boot@ system # this will allow you to edit the configuration file of the boot@ system service. Set inside the MNT_PTS variable
# 66-enable -t boot -F boot@ system # this apply the configuration change
then try to reboot.
Note: it's easier to have this mountpoint operational. This mountpoint is used by ssh,tty or whatever the program which need to use the console.
It's better to compile your kernel with its supports.Anyway, it's up to you.
Edit: Aaahhh Eric is here to the rescue, so don't pay too much attention to what I say :)

From chroot edit /etc/66/init.conf and switch verbosity=4 so you get maximum feedback on console.
If print on console is too small on grub.cfg you can alter the gfx line to a lower resolution so print is larger. 
  set gfxmode=1024x796x24
or 800x600 etc.

If you are using grub hit e on the menu item for that kernel take out quiet and splash from the entry before you boot.
If the console switches resolution before things freeze at least you have reached udev, mount-udev is a dependency for mount-pts but if something is wrong with mount-udev mount-pts will also fail. If mount-pts fails your system is ro so there is nothing hard to write a copy of the current log to so you can read later. I am suspecting an error in firmware missing that throws udev off.

The /run directory in obarun is a mount point for a temporary file system that is created during boot. Nothing in it is accessible after booting from another system. If you are using arch-chroot from obarun live into the installation what you will see in run is the live system's /run mounted into the chroot. If you mount the installation to say /mnt before you chroot try ls /mnt/run it will be empty.

If your /etc/fstab is ok and mount -a in chroot doesn't result in errors then something must be missing from the kernel and it can't mount file systems. Have you tried reverting back to an arch or obarun kernel that worked before?

example: 
# pacman -U  /var/cache/pacman/pkg/linux-5.3.13.1-1-x86_64.pkg.tar.xz
Thanks Eric. Stopping pts mounting got it to boot, but it became evident pretty quickly that I need pts, so I just restored my snapshot. Would you happen to know which part of the kernel config gives support for mounting pts though? I haven't been able to find anything mentioning pts at all, so I assume it's under a different name.
look for something like CONFIG_UNIX98_PTYS(i think is the good one) or CONFIG_DEVPTS_FS

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