Device unable to boot

[t4_4t]

> But I still tried to see if I would get anything out of serial connection even without any SD card,
> and I just get the same goop as when the SD card is in.

If your report above is similar to the photo you presented in the first report,
It means the symptom that "Tx outputs a lot of garbage even though no bootable device is attached".

At least this isn't normal.
Even if there is a baud rate mismatch,
when resetting or powering on, garbage of about 1 or 2 characters appears.
It is a symptom of that degree at best. (If there is no hardware dameges)

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"Some dirty data was unintentionally written to SPI-Flash."
It looks like that to me.

For the RK3399, SPI-Flash has the highest boot priority.
*) SPI-Flash-> eMMC-> SDcard
Therefore, if invalid data is written here,
that media will not be referenced even if a valid image is written to SDCard / eMMC.

SPI-Flash can be easily disabled with a jumper.
Does the above operation change the state ?
I recommend the above confirmation first.

The current situation is that you don't even have the minimum debugging tools,
which is a stalemate.