3 days in, slow/stuck or no boot.

[wdt]

Probably the voltage select problem is from pmOS, that's a guess, if I was betting, that's where I would put money
You have a emmc adapter, is it usb or sd? If sd, do you have a card reader?
The reason manjaro does not boot is likely that the emmc (one of the partitions) needs a fsck (do both)
(the other problem is manjaro's uboot is still defective months later, has problems booting sd, this is a long standing problem)
I assume you have another computer, probably a windows box?
Do you know how/are capable of making this (other computer) do a "linux live boot"?
Try an armbian image on a sd card
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Ordinarily the pbp cannot boot from usb, that you could was because you had tow-boot on SPI
The boot loader in the cpu is really limited, only searches SPI, emmc, sd (in that order) to find idbloader,
which initializes memory and then has enough (mem) to load uboot, uboot then alters boot search order
Likely, when you wiped SPI, the next idbloader (and uboot) is on emmc, manjaros uboot which has problems booting sd
And that is likely why you get no boot, I bet if you try to boot with the emmc removed, everything would just work
If my guess is right, then reinstall towboot to SPI