I took the Slackware current ISO and wrote it to a thumb drive.
i.e. "dd if=file.iso of=/dev/sda status=progress"

I booted the ISO-on-flash, that part was fine, but when I ran setup,
it wasn't able to auto-locate the installation files on /dev/sda.

So I tried manually mounting /dev/sda onto /u and specifying
a pre-mounted installation directory at /u.

Then setup presented the usual list of package types to install
e.g. a, ap, d, etc. but when I pressed Enter to begin the
installation, nothing happened. It skipped over all of the packages.

IIRC where sda is the target and sdb1 is the USB disk, I'd mount /dev/sdb1 /floppy and specify /floppy/slackware64 as source dir.
But I only did this a few times over last 5 years, so it's possible that I specified /floppy/ as DVD instead.
You're correct that in some cases it will not work and just skip all, took me a few times to get it right.
Note that when it fails like that, you may need to mount /dev/sda /mnt again (if sda is the target).
Others may have a better clue on what went wrong.

It turns out that the "USB stick" option works. Problem solved. Totally didn't see that.

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