I'd like to figure out how to do an upgrade with the ability to rollback fully if anything goes wrong.
Taking into account the info from the following topics:
Today I was honored to give 𝗭𝗙𝗦 𝗕𝗼𝗼𝘁 𝗘𝗻𝘃𝗶𝗿𝗼𝗻𝗺𝗲𝗻𝘁𝘀 𝗥𝗲𝗹𝗼𝗮𝗱𝗲𝗱 talk at the 𝗡𝗟𝗨𝗨𝗚 𝗔𝘂𝘁𝘂𝗺𝗻 𝗖𝗼𝗻𝗳𝗲𝗿𝗲𝗻𝗰𝗲 𝟮𝟬𝟭𝟴 (@nluug).
You may find the PDF slides useful as they contain information about upcoming bectl in FreeBSD 12.x and also mini-howto at end on how to install FreeBSD with root on ZFS on top of GELI...
New bectl tool imported to FreeBSD base as replacement for beadm tool from FreeBSD Ports.
I have made small comparison in New ZFS Boot Environments Tool article.
The bectl is already available in 12.0-ALPHA2 image...