Using /home/_sysupgrade for the temporary files strikes me as unusual to see in OpenBSD base. Is this to keep those files in the place most likely to be a separate disk partition by default installer choices?
I’d imagine it is also (mostly?) about the available disk space.
There was (at one point) some logic in sysupgrade that checked whether or not /tmp had sufficient space (and if so, it would store it there). It was a bit flimsy though, and broke when tmp was mounted as mfs. It ended up being removed.
Tangentially related: The main barrier to me moving to OpenBSD is that they don’t support extended attributes, which is a shame because otherwise it’d be the first BSD on my list to test