Finally found some time to upgrade my Pinebook Pro, these are just a few notes I made to myself, thought I'd share if anyone else was aiming for the same finish line as me.

In a nutshell
Re-flashed the SPI ROM the "hard way"
Changed the microSD boot card to something smaller, post-installation.
Changed to booting the EMMC module, after that.

Had previously been running Slackware 14.2-32-bits, installed about a year ago, had done an SPI flash already. Had left it on a shelf for ages, system was having difficulty booting anything, from SD or EMMC, would take up to 25 seconds for the power LED to change from red to green, might or might not boot the SD card. Eventually, all I could get was a red LED at the power light above the keyboard.

Attempted to re-flash the SPI with the latest SPI image, by holding the recovery switch, clicking the reset switch, and releasing the recovery switch after a few seconds. Did this many times, no luck.

Had to pull the metal cover off the mainboard, use a paper clip (Gorilla-taped to prevent grounding through my skin) to electrically short across VSS & CLK on the flash boot SPI ROM. To perform this task, I inserted the microSD card containing the flash image, removed the base of the laptop, removed the mainboard metal cover, opened the laptop to 90 degrees and laid it on its right side. Plugged it in, carefully set the paper clip on its two points on the SPI ROM, turned on the power, and waited for the prompt telling me to remove the "short". Pulled the paper clip, and it reflashed the SPI, everything went quite smoothly after that. Note - anyone with a 3D printer, it might be beneficial to have something that clips over the SPI ROM with an embedded wire, to make that precarious effort a little less dramatic. Maybe such a thing already exists?

I had an Intel 660P 2TB nvme drive in place, pre-partitioned. I use a massive (100GB) swap space, because my understanding (perhaps incorrectly) of the way writes are performed on SSDs, is that the onboard logic will try to write to the least-used memory cells, in an effort to keep them from failing quickly. So, with a small swap space and a lot of swapping, those cells will be rewritten often, even with the logic trying to mitigate against it. But, with a huge swap partition, those rewrites will get distributed across the larger partition, prolong the health of the SSD for a longer period of time. With huge SSDs at reasonable prices these days, an bunch of extra gigabytes seems prudent, to me, for the swap space. Partition 2 is a standard ext4 partition.

The installer ran fine, set up the root partition on the nvme drive and finished by releasing the non-/boot space on the microSD card. The card I used for installation was 64GB, and I have use for that card elsewhere. I have a bunch of low-capacity cards at my disposal, and the boot card only needed 331M of space, so opted to make one of the low-capacity cards a boot card.

Shut down the Pinebook, pulled the boot microSD card, and took a look at it on another x86 Linux machine. Pretty straightforward stuff. I mounted the card, and copied its contents into a temporary directory:
cp -vaR /mnt/point/of/boot/card/* /temporary/directory/

Unmounted the boot card, put a smaller card in, partitioned it with one partition, type linux(83), and made it bootable. I believe it needs a DOS partition table, so use fdisk to make that first, if you are usually using cfdisk/gparted/other to create your partitions. Made the filesystem on that card:
mkfs.ext4 /dev/sd-1

Mounted it and copied the contents of the original boot disc back onto it:
cp -vaR /temporary/directory/* /mnt/point/of/new/card/

Unmounted the new card, stuck it in the Pinebook Pro, and it boots up just fine. One useless microSD card used up, one good one ready for more action.

I then did the same for an EMMC drive. No matter what I did, I could not get the system to see my shiny new 128GB EMMC module. Stuck it in a USB converter, inserted it into my x86 workstation, and found it had a GUID partition table. Used fdisk to create a DOS partition table, created two partitions on it (2G & 126GB IIRC). made the first bootable, gave both an ext4 filesystem, made the filesystems, mounted the first partition, and copied the boot files over to it, just like I did with the smaller microSD card.

Shut down the Pinebook Pro again, opened it up again, installed the EMMC module, and changed the switch to boot from EMMC. Re-assembled the system, turned it on, and everything is kewlio. System boots from the EMMC drive, mounts the nvme drive as /, and leaves me at a Slackware command prompt and a free microSD slot.

Side note - I had an A-DATA XPG SX8100 4TB drive in the nvme slot, while I was initially troubleshooting, and the odd time I was able to get the laptop booted up, that drive seemed to work fine. I may do a long-term installation, eventually.

Thanks drmozes and all the Slackware team, I'll send some more coin your direction later on tonight. I do not have time to haunt the forums, so if you have questions/comments about the post, do not hope for quick answers (from me at least).