Pinebook Pro doesn't boot most of the time unless connected via USB-serial!

[wdt]

Not so much info on MODERN uboots, often you see pages dated <2010
I would do a whole distro, in case your systemd setup wrong(or dtb), then try
dd-ing its 1st 16M to a save, then to your minimal card (save/restore mbr)
(I'll explain more, it is really easy to, from beginning(of media), copy or write to media,
here 16M, mbr will be overwritten, so save original as 1st step, restore as last step,,,
more simple, just bs=1M count=16 ,, (as well as if=,of=) ,, no seek, no skip)
If you do mrfixit (my recomendation), go to his git for most up to date, then update it,,mrfixit_update.sh)
Yes indeed, you can mix and match, as long is hardware is same, for that matter,
when you boot SD, it will be using uboot from emmc, not the one on the SD, so may well be mixed
A little more,, bootrom search order is SPI, emmc, SD. usb for rkdeveloptool.
1st uboot found (really idbloader) is loaded, all other uboots ignored

uboot has its own search order (strings uboot|grep _target), looking for extlinux/boot.scr
uboots search will nearly always be different, an example (samueldr)
boot_targets=mmc1 usb0 nvme0 mmc0 pxe dhcp sf0 ,,,, mmc1 is SD, mmc enumerated in order found