I'm writing the post I wish I would have found when I ran into this problem. It already includes the answer that worked for me.
I re-imaged the Raspberry Pi Zero 2 W installed directly on the Einsy RAMBo main board on my Prusa i3 MK3S printer. No serial ports were detected.
I had forgotten what I had to do in order to get that serial-via-GPIO connection working. Fortunately I reimaged a new microSD card so I could compare my new image with a working image.
To get the serial via GPIO working you must make two edits:
console=tty1 root=PARTUUID <YOUR_PARTITION_ID> rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait
After rebooting the printer was automatically detected on
/dev/ttyS0 at 115200 baud.
There are other forum posts on the internet that change the UART and whatnot, but I couldn't get them to work. I have been using the default/inferior UART for years without issue.