Anyone planning on ZFS or BTRFS for file systems?

[DrYak]

Now I have to decide, (though it will probably take me weeks), whether to use the whole eMMC, (except for "/boot" and swap), as BTRFS. Then control the sub-volumes for things like "/home", (which I would use my snapshot script).

That's literally been my modus operandi on my pinebook pro and several other devices (e.g.: my workstation at work, etc.), and also been the default suggestion in openSUSE (and also on CentOS back when they still used BTRFS for snapshots, before abandoning it and moving to stratis).
Basically keep your root in one subvolume (the default one) and have everything else that you don't want to version (like home, opt, usr/local, srv, tmp, various entries in var, etc.) in a different subvolume that you won't snapshot.

A long time ago, I used to have a separate /home because:

• out of habit
  
• space management (overfilling /home won't prevent the system to run with an over filled /)
  (that was an important point before ENOSP got fixed).
  
• when BTRFS was young, out of fear of losing my /home if / got corrupted
  (Turns out: never got any major problem with BTRFS that couldn't be recovered by CoW -- or in the case of dead SD card, by btrfs restrore)
  

Nowadays, the only still valid reasons to use multiple partition is that currently you can't specify per-subvolume RAID levels (yet. it's on the devs todo list).
So if you want your root's data chunks at RAID0 for speed and your home's data chunks at RAID1 for resilience, multiple partitions is the only way to go.
(I do that on my working laptop and my homeworkstation).

But that requires multiple devices (or in case of "dup" a spinning rust HDD).

As you only have a single eMMC in your PBP, the whole point is moot.
(well, unless you manage to fit dual NVMe and a PXL chip within the form factor and the power envelope of the pbp ...)

And snapper is not designed for alternate boot environments.

On the other hand, there were some people on this forum toying with the idea of booting grub, and snapper does support grub. (It's been also be toyed by o-droid users, and even Suse made conferences about it)

@DrYak, thanks for the pointer to snapper, but it will be easy enough to modify my old ZFS snap shot utility.

Yup, we all like our home-made tools, hand carved with love. And with Bash/Perl/whatever.